ELEMENTS  OF 
PHYSIOLOGY 

V 

HOUGH  AND  SEDGWICK 


ELEMENTS  OF  PHYSIOLOGY 


BEING  PART  I  OF  "THE  HUMAN  MECHANISM 

ITS  PHYSIOLOGY  AND  HYGIENE  AND 

THE  SANITATION  OF  ITS 

SURROUNDINGS" 


BY 
THEODORE  HOUGH 

Professor  of  Physiology  and  Dean  of  the  Department  of 
Medicine  in  the  University  of  Virginia 

AND 

WILLIAM  T.  SEDGWICK 

Professor  of  Biology  and  Public  Health  and  Lecturer  on  Hygiene 
and  Sanitation,  Massachusetts  Institute  of  Technology 


R  E  VIS  K  I)  EDI  77  ON 


GINN  AND  COMPANY 

BOSTON    •     NEW   YORK    •    CHICAGO    •     LONDON 
ATLANTA    •     DALLAS    •     COLUMBUS     •     SAN   FRANCISCO 


BIOLOGY 

LIBRARY 

G 


ENTERED  AT  STATIONERS'  HALL 


COPYRIGHT,  1906,  1918,  BY 

THEODORE  HOUGH  AND  WILLIAM  T.  SEDGWICK 

ALL  RIGHTS  RESERVED 
420.11 


gfte 


GINN  AND  COMPANY-  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PREFACE  TO  THE  REVISED  EDITION 

This  edition  presents  a  thorough  revision,  in  which  the 
authors  have  incorporated  /  those  advances  in  physiology 
which  are  directly  applicable  to  the  fundamental  purpose 
of  this  book  as  stated  in  the  preface  to  the  first  edition. 
Portions  of  certain  chapters  have  been  entirely  rewritten,  — 
notably  those  dealing  with  the  work  of  organs  and  cells, 
internal  secretions,  digestion,  nutrition,  and  the  central 
nervous  system. 

The  advances  of  physiological  knowledge  in  the  past 
decade  have  not  only  given  clearer  insight  into  the  nature 
of  the  processes  which  underlie  the  phenomena  of  living- 
things  but  have  also  made  the  facts  of  physiology  increas- 
ingly helpful  in  the  intelligent  conduct  of  life.  While  this 
latter  point  of  view  has  chiefly  determined  the  selection  of 
material  to  be  included  in  this  book,  it  has  none  the  less 
been  necessary  to  lay  the  foundation  for  the  understanding 
of  what  we  call  the  practical  knowledge  by  a  clear  and  suc- 
cinct statement  of  the  fundamental  principles  of  physiology. 
This  first  part  of  "  The  Human  Mechanism  "  therefore  serves 
the  purpose  of  those  who  cannot  give  the  time  necessary 
for  the  more  extensive  study  of  physiology  required  of  the 
physician  or  the  specialist  in  physiological  science. 

It  is  hoped  that  the  interest  aroused  in  the  applications 
of  physiology  to  the  conduct  of  life  may  lead  many  readers  of 
this  book  to  the  subsequent  study  of  hygiene  and  sanitation. 
The  second  part  of  "  The  Human  Mechanism"  has  therefore 
been  published  under  the  title  "Hygiene  and  Sanitation." 

We  are  indebted  to  Dr.  E.  P.  Joslin  for  permission  to 
reproduce  from  his  work  on  "  The  Treatment  of  Diabetes 
Mellitus"  the  table  on  page  238. 

p  iii 

518533 


PREFACE 

The  present  book  is  a  reprint  of  the  physiological  por- 
tion of  our  larger  work  entitled  The  Human  Mechanism, 
together  with  Chapter  XVI  (Drugs,  Alcohol,  and  Tobacco), 
which  has  been  added  to  meet  the  requirements  of  law 
in  some  states  with  regard  to  the  teaching  of  physiology. 
For  those  who  desire  in  compact  form  the  elements  of 
physiology  as  a  part  of  general  biological  training,  as  an 
introduction  to  the  study  of  psychology,  or  for  other  spe- 
cial purposes,  and  for  those  who,  having  undertaken  the 
study  of  hygiene  and  sanitation  in  Elements  of  Hygiene 
and  Sanitation  (Part  II  of  The  Human  Mechanism),  desire 
to  acquaint  themselves  more  fully  with  the  fundamental 
physiology,  the  present  volume  should  prove  useful. 

The  references  to  Part  II  have  been  retained  in  the  text, 
and  apply  either  to  The  Human  Mechanism  or  to  Elements 
of  Hygiene  and  Sanitation. 


CONTENTS 

CHAPTER  PAGE 

I.   THE  HUMAN  MECHANISM    .     , .  .     . 3 

II.   THE  STRUCTURE  (ANATOMY)  OF  THE  HUMAN  MECHANISM  6 

III.  THE    FINER    STRUCTURE    OF    Two    TYPICAL    ORGANS, 

GLANDS  AND  MUSCLES.    THE  CONNECTIVE  TISSUES. 

THE  LYMPHATIC  SYSTEM 28 

IV.  THE  ORGANS  AND  CELLS  OF  THE  BODY  AT  WORK   .     .  43 
V.   WORK  AND  FATIGUE  ....'...;,.....  55 

VI.   THE    INTERDEPENDENCE    OF    ORGANS    AND    OF    CELLS. 

INTERNAL  SECRETIONS.     .     .     ......     .     .     J     .  63 

VII.   THE  ADJUSTMENT  OR  COORDINATION  OF  THE  WORK  OF 

ORGANS  AND  CELLS      ...,...*.;.  69 

VIII.   ALIMENTATION  AND  DIGESTION    .     .     .     .     .     .  ,  .     N    .  91 

The  Supply  of  Matter  and  Power  to  the  Human  Machine  91 

Digestion  in  the  Mouth.  Enzymes  . 103 

Digestion  in  the  Stomach 107 

Digestion  and  Absorption  in  the  Small  Intestine  and  in 

the  Large  Intestine 117 

IX.   THE  CIRCULATION  OF  THE  BLOOD    .     . 135 

Blood  and  Lymph      . ' 135 

Mechanics  of  the  Circulation  of  the  Blood  and  of  the 

#      Flow  of  Lymph '. 139 

The  Adjustment   of   the   Circulation  to  the  Needs  of 

Everyday  Life  . 152 

X.   RESPIRATION 165 

XL    EXCRETION      .............'...  180 

XII.   THERMAL  PHENOMENA  OF  THE  BODY 189 

The  Constant  Temperature 189 

The  Regulation  of  the  Body  Temperature 199 

1*  vii 


viii  ELEMENTS  OF  PHYSIOLOGY 

CHAPTER  PAGE 

XIII.  NUTRITION 215 

The  Sources  of  Power  and  Heat  for  the  Human 

Mechanism 215 

The  Food  Reserve  of  the  Body.  Fat.  Glycogen.  Cell 

Proteins 222 

Food  as  the  Material  for  Growth,  Repair,  and  the 

Manufacture  of  Special  Products  of  Cell  Activity  229 

The  Proper  Daily  Intake  of  Protein 235 

XIV.  SENSE  ORGANS  AND  SENSATIONS 240 

XV.   THE  NERVOUS  SYSTEM 263 

Its  Anatomical  Basis 263 

The  Physiology  of  the  Nervous  System 269 

XVL   FOOD  ACCESSORIES,  DRUGS,  ALCOHOL,  AND  TOBACCO  .  286 

INDEX  .  309 


THE  HUMAN  MECHANISM 
PART  I 

ELEMENTS  OF  PHYSIOLOGY 


PAET  I 

CHAPTER  I 
THE  HUMAN  MECHANISM 

1.  The  human  body  a  living  organism.  The  human  body, 
as  compared  with  bodies  of  water  such  as  lakes  and  seas, 
or  with  heavenly  bodies  such  as  the  sun,  moon,  and  stars, 
is  a  small  mass  of  matter  weighing  on  the  average,  when 
fully  grown,  about  150  Ib.  and  measuring  in  length  about 
5  ft.  9  in.  It  is  neither  very  hot,  as  is  the  sun,  nor  warm 
in  summer  and  cold  in  winter,  as  are  many  bodies  of 
water,  but  in  life  and  health  has  always  almost  exactly  the 
same  moderate  temperature,  namely,  98.6°  F.  or  37.5°  C.  The 
human  body  is  not  homogeneous,  that  is  to  say,  alike  in  all 
its  parts,  as  is  the  substance  of  a  lake,  but  consists  of  very 
unlike  parts  —  eyes,  ears,  legs,  heart,  brain,  muscles,  etc.  — 
these  parts  being  known  as  organs,  and  the  whole  body,  there- 
fore, as  the  human  organism. 

The  most  remarkable  peculiarity  of  the  human  body,  how- 
ever, is  that  it  is  a  living  organism.  A  watch  has  unlike  parts 
—  spring,  dial,  hands,  case,  etc.  —  which  are  essentially  its 
organs,  and  the  watch  might  therefore  be  called  an  organism; 
yet  it  never  is  so  called.  We  speak  of  a  well-organized  army, 
navy,  government,  society,  church,  or  school,  but  never  of  a 
well-organized  automobile,  typewriter,  printing  press,  or  loco- 
motive —  apparently  for  the  reason  that  in  army,  navy, .  or 
school  living  things  play  a  principal  part,  while  in  mere 
machinery  life  is  wholly  wanting.  The  highest  compliment 
we  can  pay  to  a  machine  is  to  say  that  it  seems  "almost 

3 


:  MECHANISM 


alive,"  but  it  is  not  a  compliment  to  any  human  being  to 
describe  him  as  "  a  mere  machine."  What  the  vital  property 
is,  what  we  mean  by  the  terms  "  life  "  and  "  living,"  no  one 
can  exactly  tell.  About  all  we  know  of  it  is  that  some  of  the 
commonest  elements  of  matter  (carbon,  hydrogen,  oxygen, 
and  nitrogen,  with  a  little  sulphur,  phosphorus,  and  a  few 
other  elements)  frequently  occur  combined  as  living  matter, 
and  that  this  living  matter  has  marvelous  powers  of  growth, 
repair,  and  reproduction,  besides  a  certain  spontaneity,  origi- 
nality, and  independence,  which  lifeless  matter  never  displays. 
"  While  there  is  life  there  is  hope  "  for  any  plant  or  any  ani- 
mal, but  this  saying  does  not  apply  to  any  lifeless  machine, 
however  complex  or  wonderful. 

2.  The  human  body  a  living  machine  or  mechanism.  By 
a  machine  we  mean  an  apparatus,  either  simple  or  complex, 
and  usually  composed  of  unlike  parts,  by  means  of  which 
power  received  in  one  form  is  given  out  or  applied  in  some 
other  form.  This  power  may  be  received,  for  example,  in  the 
form  of  heat,  or  electricity,  or  muscular  effort,  or  as  the  poten- 
tial energy  of  fuel  ;  and  it  may  be  given  out  as  heat,  or  elec- 
tricity, or  light,  or  sound,  or  as  mechanical  work,  or  in  any 
one  of  many  other  ways.  One  of  the  simplest  of  all  machines 
is  a  stove,  an  apparatus  composed  of  a  few  simple  parts  by 
means  of  which  the  potential  energy  or  power  of  fuel  — 
wood,  coal,  gas,  or  oil  —  is  liberated  and  applied  as  heat,  for 
warming  or  cooking.  A  lamp  is  a  still  simpler  machine  in 
which  the  potential  energy  or  power  of  gas  or  oil  is  liberated 
and  converted  into  useful  light.  A  candle  is  a  lamp  so 
simple  that  it  almost  ceases  to  be  a  machine,  and  yet  the 
wick  is  really  an  apparatus  for  securing  proper  combustion 
of  wax  or  tallow  to  provide  good  light. 

Machines  of  greater  complexity  are  watches  or  clocks, 
pieces  of  apparatus  composed  of  many  unlike  parts  which 
receive  power  in  comparatively  large  amounts  for  a  short  time 
during  the  process  of  winding,  store  it  as  potential  energy  in 


THE  HUMAN  MECHANISM  5 

coiled  springs  or  lifted  weights,  and  liberate  it  slowly  in  the 
mechanical  work  of  moving  the  hands  of  the  timepiece  over 
a  dial.  Still  more  complex  is  a  locomotive  or  an  automobile, 
machines  in  which  the  power  of  coal,  oil,  gasoline,  or  other 
fuel  or  the  electricity  of  a  storage  battery  is  applied  to  swift 
locomotion.  But  the  most  wonderful  of  all  machines  is  the 
human  body,  a  complicated  piece  of  apparatus  in  which  the 
power  stored  in  foods,  such  as  starch,  sugar,  butter,  meat, 
milk,  eggs,  and  fish,  is  transformed  into  that  heat  by  which 
the  body  is  warmed  and  into  that  muscular,  nervous,  diges- 
tive, or  other  work  which  it  performs. 

For  delicate  and  intricate  machinery  the  term  "  mechan- 
ism "  is  often  employed,  and  we  may  therefore  describe 
the  human  body  either  as  the  "  human  organism,"  or  the 
"  human  machine,"  or,  perhaps  best  of  all,  as  the  HUMAN 

MECHANISM. 

The  study  or  the  science  of  the  construction  (structure) 
of  this  mechanism  is  called  its  anatomy;  of  its  ordinary  be- 
havior, operation,  or  working,  its  physiology;  of  its  proper 
management,  protection,  and  care,  its  hygiene.  This  textbook 
is  devoted  chiefly  to  an  account  of  its  operation  and  care, 
that  is,  to  its  physiology  and  hygiene ;  but  as  any  true  com- 
prehension of  these  subjects  depends  upon  some  preliminary 
knowledge  of  the  parts  of  the  mechanism  itself,  we  shall 
begin  by  considering  briefly  the  structure  or  anatomy  of  the 
human  machine. 


CHAPTER  II 

THE  STRUCTURE  (ANATOMY)  OF  THE  HUMAN 
MECHANISM 

Anatomy  is  studied  partly  by  dissection,  which  reveals 
chiefly  those  organs  which  are  visible  to  the  naked  eye,  and 
partly  by  microscopic  examination,  which  gives  a  deeper  in- 
sight into  the  detailed  arrangement  of  the  cells  and  tissues 
of  which  the  organs  of  the  mechanism  are  composed.  The 
present  chapter  is  devoted  to  structures  or  organs  shown 
by  dissection  —  the  gross  anatomy  of  the  body  —  as  distin- 
guished from  its  microscopic  anatomy  (histology).1 

1  Further  explanation  of  the  structure  of  the  human  machine  will  be 
given  as  it  may  be  needed  in  subsequent  chapters.  At  this  point  it  is  of 
the  utmost  importance  that  the  student  thoroughly  master  the  general 
relations  of  the  more  important  organs  one  to  another ;  this,  however, 
is  not  to  be  done  by  extensive  reading,  and  still  less  by  memorizing  verbal 
descriptions ;  the  aim  should  rather  be  to  acquire  from  figures  and  dia- 
grams, or  better  yet  from  actual  dissection,  where  that  is  possible,  a  correct 
mental  picture  of  the  structures  involved.  Far  more  can  be  learned  by 
constructing  drawings  or  diagrams  from  memory  than  by  the  mere  memo- 
rizing of  text.  The  drawings  may  lack  finish  and  may  be  at  first  difficult 
to  execute  ;  but  so  long  as  they  represent  the  relations  of  the  organs  one 
to  another  they  accomplish  their  purpose ;  beyond  this  point  the  more 
accurately  they  are  drawn  the  better. 

Moreover,  drawing  is  a  great  aid  to  dissection.  It  not  only  fixes  in  the 
memory  what  is  seen  but  it  compels  close  observation  ;  when  one  draws  an 
object  he  is  forced  to  note  details  and  relations  of  structure  which  would 
otherwise  escape  observation.  Nor  is  the  freehand  drawing  which  is  re- 
quired for  our  purpose  so  difficult  as  is  often  supposed  by  those  who 
have  never  seriously  used  it.  Let  the  student  attempt  to  reproduce  an 
object  from  his  memory  of  its  picture ;  begin  with  one  which  is  not  too 
complicated  (such  as  the  figure  of  the  peritoneum  and  mesentery  on 
page  14).  Where  he  does  not  know  how  to  represent  a  special  structure, 
let  him  refer  to  the  original,  from  which  he  may  get  suggestions;  then 
close  the  book  and  draw  from  memory ;  any  completed  part  of  the  work 
may  be  compared  with  the  original  and  possible  improvements  discovered. 

6 


STEUCTUKE  OF  THE  HUMAN  MECHANISM         7 

The  human  mechanism  is  composed  of  different  parts,  such 
as  head,  neck,  trunk,  arms,  hands,  legs,  and  feet,  and  each  of 
these  in  its  turn  is  composed  of  lesser  parts.  Arms  and  hands, 
for  example,  are  covered  by  skin,  which  may  be  moved  over 
underlying  soft  parts ;  at  the  ends  of  the  fingers  the  place  of 
the  skin  is  taken  by  nails,  while  scattered  over  and  emerging 
from  its  surface  are  hairs.  Through  the  skin  may  be  seen  the 
veins,  which  may  be  emptied  of  the  purplish  blood  they  con- 
tain by  pressing  one  finger  on  a  part  of  the  vein  near  the 
finger  and  pushing  another  finger  along  the  vein  toward  the 
wrist ;  so  long  as  pressure  is  maintained  by  both  fingers 
the  vein  remains  collapsed,  but  on  removing  the  first  finger 
it  fills  again  with  blood.  Finally,  through  the  soft  parts 
(flesh)  may  be  felt  the  hard  bones.  In  general  these  various 
parts  of  which  the  body  is  composed  are  known  as  its  organs, 
and  because  it  possesses  organs  it  is  called  an  organism  (p.  3). 

1.  The  skin.    The  body  is  everywhere  covered  by  a  com- 
plex protective  and  sensitive  organ,  the  skin.    Only  the  eyes 
and  nails  seem  to  be  exceptions ;  but  as  a  matter  of  fact  the 
exposed  surface  of  the  eye  is  covered  by  a  very  thin,  trans- 
parent portion  of  the  skin,  and  the  nails  are  really  modified 
portions  of  skin. 

2.  Subcutaneous  connective  tissue.    On  cutting  through  the 
skin  we  find  that  it  is  bound  to  the  underlying  flesh  (chiefly 
meat  or  muscle)  by  what  is  known  as  connective  tissue,  the 
structure  of  which  we  shall  study  in  the  next  chapter.  Mean- 
while we  may  notice  that  it  contains  blood  vessels,  that  at 
some  places  it  is  more  easily  stretched  than  at  others,  and 
that  when  a  flap  of  skin  is  pulled  away  from  the  muscles, 
this  subcutaneous  tissue  fills  with  air.   It  often  contains  large 
quantities  of  fat. 

Such  practice  may  well  precede  drawing  from  an  actual  dissection  and 
will  pave  the  way  to  the  latter.  At  all  events  let  the  student  understand 
thoroughly  that  in  the  present  chapter  the  figures,  supplemented  if  possible 
by  actual  dissections,  form  the  main  objects  of  study ;  the  text  is  strictly 
subordinate  to  the  figures. 


8  THE  HUMAN  MECHANISM 

3.  Muscles    and    deeper   connective   tissues.1     The   subcu- 
taneous connective  tissue  sometimes  connects  or  binds  the 
skin  directly  to  bone,  as  in  parts  of  the  head ;  usually,  how- 
ever, in  the  neck,  trunk,  and  limbs  the  underlying  tissue  is 
the  red  flesh,  or  muscle,  familiar  to  us  as  "  lean  of  meat." 
If  the  skin  be  removed  from  the  forearm,  it  at  once  becomes 
evident  that  this  mass   of  meat  or  flesh  is  composed  of  a 
number  of  muscles  which  may  be  separated  from  one  an- 
other more  or  less  completely.    In  doing  this  it  will  be  found 
that  the  muscles  are  held  together  by  connective  tissue  in 
most  respects  quite  similar  to  that  immediately  under  the 
skin.    Further  dissection  will  show  that  one  or  another  form 
of  this  tissue  is  the  means  of  binding  other  organs  together ; 
thus  the  muscles  are  joined  to  the  bones  by  a  very  dense, 
compact,  and  strong  form  known  as  tendon:,  the  bones  are 
united  by  a  somewhat  similar  r.rm  known  as  ligament;  and 
so  on.    The  physical  characters  of  the  tissue  differ  widely, 
according  to  its  situation  and  the  use  subserved;  but  one 
form  shades  more  or  less  into  another,  and  we  have  no  diffi- 
culty in  recognizing  the  general  similarity  which  leads  us  to 
group  them  all  together  in  one  class. 

4.  Muscles  attached  to  bones.    When  a  muscle  is  carefully 
dissected  away  from  neighboring  muscles  and  other  organs, 
it  is  almost  always  found  that  it  is  attached  to   one  and 
usually  to   two  bones;   this   union  is  frequently   made   by 
means  of  a  tendon,  as  in  the  case  of  the  large  muscle  of  the 
calf  of  the  leg,  which  is  attached  at  one  end  to  the  bone 
of  the  thigh  and  at  the  other  to  that  of  the  heel.    A  good 
example   of  the  direct  attachment   of   muscles  to   bones  is 
furnished  by  those  muscles  which  lie  between  the  ribs  (see 


1  The  general  appearance  and  arrangement  of  muscles,  their  attachment 
by  means  of  tendons  to  bones,  and  the  action  of  tendons  on  bones  can  be 
beautifully  shown  by  a  dissection  of  the  leg  of  a  chicken.  The  difference 
between  trunk  and  limbs  in  the  matter  of  the  body  cavity  may  also  be 
readily  demonstrated  on  the  same  animal. 


STBUCTUBE  OF  THE  HUMAN  MECHANISM        9 


Fig.  161).    In  either  case  the  shortening  of  the  muscle  brings 
closer  together  the  bones  to  which  it  is  attached. 

5.  Definition  of  some  anatomical  terms.  Before  proceeding 
further  we  must  agree  upon  the  exact  meaning  of  certain 
anatomical  terms.  We  often  speak  of  one  part  of  the  body  as 
being  "above"  or  "below,"  "before"  or  "behind,"  another. 
Such  terms,  however,  are  confusing,  because  their  meaning  de- 
pends upon  the  position  of  the  body  at  the  time  they  are  used. 
For  example,  when  one  is  lying  on  his  back  the  head  is  in 
front  of,  or  before,  the 
trunk ;  but  when  he  is 
standing  on  his  feet  it 
is  above  the  trunk. 

Now  the  body  is 
certainly  divided  into 
right  and  left  halves, 
which  are  much  alike 
externally,  though  this 
likeness  is  not  so 
marked  in  the  internal 
parts.  Right  and  left 
then  have  their  ordi- 
nary meanings,  and 
that  without  regard  to 

flip     vnrinn^     nnditinn<*     ^,  skin ;  7?,  subcutaneous  connective  tissue,  bind- 
VailC  ing  the  skin  to  the  muscles  D  and  continuous 

with  the  connective  tissue  which  binds  together 
the  muscles ;  C,  blood  vessels  and  nerves 


FIG.  1.    Cross  section  of  arm 


the  body  may  take. 

To  indicate  that  any 
part  is  nearer  the  head  than  another  part,  we  say  that  the 
former  is  anterior  to  the  latter ;  to  indicate  that  the  latter  is 
further  away  from  the  head,  we  say  it  is  posterior  to  the  former. 

Finally,  the  region  popularly  known  as  the  back  is  called 
dorsal  (Latin  dorsum,  "  back  "),  that  opposite  the  back  being 
called  ventral  (Latin  venter,  "belly").  Thus  the  nose  is  on 
the  ventral  side  of  the  head;  the  toes  are  at  the  posterior 
extremity  of  the  foot. 


10 


THE  HUMAN  MECHANISM 


6.  The  body  cavities.  There  is  one  striking  and  important 
structural  difference  between  the  trunk  and  the  limbs ;  the 
former  contains  a  central  body  cavity,  completely  filled,  how- 
ever, with  various  organs,  while  the  arms  and  legs  are  each 
composed  of  a  continuous  mass  of  tissues,  namely,  muscle, 

bloodvessels,  nerves, 
bone,  etc.,  all  bound 
together  by  connec- 
tive tissue  (Figs.  1 
and  2). 

The  cavity  of  the 
trunk,  or  body  cav- 
ity, is  subdivided 
transversely  by  the 
dome-shaped  muscle 
known  as  the  dia- 
phragm into  two 
cavities  —  an  ante- 
rior, known  as  the 
thoracic,  or  pleural, 
cavity ;  and  a  poste- 
rior, known  as  the 
abdominal,  or  perito- 
neal, cavity.  Both 
cavities  are  lined  by 
a  thin,  smooth,  shiny 
membrane,  that  of 
the  thoracic  being 
known  as  the  pleura,  and  that  of  the  abdominal  as  the 
peritoneum. 

Filling  the  pleural  cavity  are  found  the  heart,  lungs, 
oesophagus,  windpipe  (or  trachea),  and  many  great  blood  vessels ; 
filling  the  abdominal  cavity,  the  stomach,  the  small  intestine, 
the  large  intestine,  the  liver,  the  pancreas,  the  kidneys,  the 
spleen,  and  other  organs,  together  with  numerous  large  and 


FIG.  2.  The  thoracic,  or  pleural,  and  the  abdomi- 
nal, or  peritoneal,  cavities  filled  with  organs 


STRUCTURE  OF  THE  HUMAN  MECHANISM      11 


important  arteries  and  veins.  In  both  cavities  the  lining 
membrane  (pleura  or  peritoneum)  is  folded  back  over  the 
organs ;  that  is  to  say,  the  organs  do  not  really  lie  in  the 
cavities,  but  only  fill  them  as  the  hand  would  fill  a  bladder 
one  wall  of  which  it  pushes  in  against  the  other.  The  sur- 
faces of  the  organs,  like  the  walls  of  the  cavity,  are  conse- 
quently smoothly  covered  and  glide  over  one  another  with 
very  little  friction.  The 
preservation  of  these  pleu- 
ral  and  peritoneal  linings 
in  their  normal  condition 
is  a  matter  of  great  impor- 
tance ;  when  inflamed  or 
otherwise  injured  their  sur- 
faces become  roughened, 
and  adhesions  of  connec- 
tive tissue  often  develop 
between  them  which  fas- 
ten the  organs  together  or 
to  the  walls  of  the  cavity,  FlG-  3-  Cross  section  of  the  chest  ante- 


so  that  surgical   interfer- 
ence  is  sometimes  neces- 


rior  to  the  branching  of  the  trachea 

A,  a.  vertebra  of  the  spinal  column ;  B,  spinal 
cord ;  (7,  the  pleural  cavity  (which  is  exag- 


\         Sary.     Pleurisy  is   Such   an  gerated  for  the  sake  of  clearness,  the  sur- 

\        .    a  ,  •  r  ,  i         i  face  of  the  lung  being  actually  in  contact 

\      inflammation  Of  the  pleura,  with  the  body  wall).    The  oesophagus,  tra- 

\  peritonitis    of    the    peritO-  chea»  together  with  several  large  arteries 

,   ,       ,  and  veins,  are  shown  in  the  mediastinum  ven- 

\neum  ;    and  both  are  Very  tral  to  the  vertebra  and  in  the  order  named 

serious  conditions. 

7.  Attachment  of  the  organs  to  the  walls  of  the  pleural 
and  peritoneal  cavities.  The  pleural  cavity  is  completely 
divided  by  a  median  partition  of  connective  tissue  (the 
mediastinum),  within  which  are  found  the  trachea,  the 
oesophagus,  the  great  blood  vessels,  and  —  lying  within  a 
special  cavity  of  its  own  —  the  heart.  Approximately  half- 
way from  the  anterior  to  the  posterior  border  of  the  medi- 
astinum the  trachea  divides  within  that  membrane  into  two 


12 


THE  HUMAN  MECHANISM 


tubes,  or  bronchi,  which  pass  through  the  mediastinum  out- 
ward, one  to  the  right  lung,  the  other  to  the  left.  The  pleural 
lining  of  the  mediastinum  is  pushed  outward  by  these  tubes 
and,  as  they  end  in  the  lungs,  forms  the  pleural  covering  of 
the  latter  (Fig.  5).  Consequently  the  organs  of  the  pleural 
cavity  either  lie  within  the  mediastinum  (heart,  oesophagus, 


FIG.  4.    Cross  section  of  chest  posterior  to  branching  of  trachea 

A,  bronchus,  entering  the  lung ;  B,  the  aorta  cut  at  its  origin  and  again  at  the 

descending  part  of  its  arch;    C,  the  pericardial  space;   D,  the  pleural  cavity; 

P. A.,  the  pulmonary  artery 

trachea,    etc.)    or   else   are    covered    by   extensions    of   the 
mediastinal  pleura  (bronchi  and  lungs). 

The  abdominal  cavity  is  not  similarly  separated  into  right 
and  left  halves ;  but  a  membrane,  the  mesentery,  passes  ven- 
trally  from  the  dorsal  wall  to  the  stomach  and  intestine, 
which  are  slung  in  it  somewhat  as  a  man  lies  in  a  hammock. 
The  line  of  attachment  of  this  mesentery  to  the  small  intes- 
tine is  much  longer  than  that  of  its  attachment  to  the  body 
wall ;  hence  it  has  the  general  shape  of  a  ruffle,  or  flounce  — 
an  arrangement  which  permits  the  suspension  of  the  very 


STRUCTURE  OF  THE  HUMAN  MECHANISM      13 


long  intestine  (20  to  25  ft.)  from  the  comparatively  short 
median  dorsal  body  wall  (see  Fig.  156).  The  great  arteries 
and  veins  lie  in  the  mesentery  near  the  dorsal  body  wall, 
and  branches  are  distributed  from  them  to  the  intestine 
within  this  expand- 
ing membrane  (see 
Fig.  163). 

The     kidneys     do 
not  He  movably  sus- 
pended   in     the    ab- 
dominal cavity,  as  do 
the     intestines,     but 
are  large  organs,  one 
on  each  side,  situated 
near  the  spinal  column 
and  dorsal  to  the  ab- 
dominal  cavity  from 
which  they  are   sep- 
arated by  the  peri- 
toneum.    Arteries 
and  veins  are  sup- 
plied to  them  from 
the    large    median 
artery  and  the  me- 
dian  vein    already 
referred  to    (aorta 


cava 


and       vena 

Fig.  15),  and  these 

renal  arteries  and  veins  are 

likewise  outside-  the  abdom- 


5.    Diagrammatic  vertical  right- 
to-left  section  of  the  right  thorax 

A,  muscles,  ribs,  etc.,  of  the  body  wall ; 
JB,  pleura,  lining  the  same ;  C,  the  pleural 
space  or  cavity ;  D,  the  pleural  covering 
of  the  lung ;  E,  connective  tissue  of  the 
lung;  F,  alveoli  of  the  lung;  G,  dia- 
phragm ;  H,  trachea ;  7,  right  bronchus, 
branching;  K,  the  pericardial  space  in 
which  lies  the  heart.  Note  the  division 
of  the  lung  into  two  lobes 


inal  cavity. 

The  relation  of  the  other  organs  to  the  peritoneum  is  more 
complicated,  notably  in  the  case  of  the  liver ;  but  in  all  cases 
the  organs  are  inclosed,  or  wrapped,  either  in  a  fold  of  the 
peritoneum,  as  is  the  kidney,  or  in  a  fold  of  the  mesentery, 


THE  HUMAN  MECHANISM 


as  is  the  intestine;  and  their  blood  and  nerve  supplies  run 

to  them  in  similar  folds. 

8.  The  axial  skeleton.    The  bones  and  cartilages  of  which 

the   skeleton  is   composed   may  be   classified    into  an   axial 

skeleton  (of  the  head, 
fKidney  neck,  and  trunk)  and 
an  appendicular  skele- 
ton (of  the  arms  and 
legs).  The  axial  skele- 
ton comprises  (1)  the 
backbone,  or  vertebral 
column,  (2)  the  ribs 
and  breastbone,  and 
(3)  the  skull. 

9.  The  backbone, 
or  vertebral  (spinal) 
column.  This  is  com- 


FIG.  6.    Diagrammatic   cross   section   of   the 
abdominal  cavity 

Showing  the  relation  of  the  kidneys  and   great 
blood  vessels  to  the  peritoneum.  The  intestine  has 


been  removed,  the  cut  border  of  the  mesentery     posed    of   Separate   ir- 


regular ringlike  bones, 


being  shown 

or  vertebrae,  placed  one  above  another  and  bound  together  by 
bands  of  strong  connective  tissue  known  as  ligaments.  It  is 
customary  to  divide  the  backbone  into  the  following  regions : 

Cervical,  7  vertebrae  of  the  neck. 

Thoracic,  12  vertebrae  of  the  chest,  to  which  ribs  are  attached. 

Lumbar,  5  vertebrae  of  the  "  small  of  the  back." 

Sacral,  5  vertebrae  (fused  together)  to  which  the  large  hip  bones 
are  attached. 

Coccygeal,  4  or  5  very  small,  simple  vertebrae  (constituting  the  skele- 
ton of  a  rudimentary  tail  and  corresponding  to  the  tail  of  lower 
animals). 

When  one  looks  at  the  spinal  column  from  behind,  the 
vertebras  are  seen  to  be  placed  one  upon  another,  but  all  in 
the  median  dorsoventral  plane  of  the  body  (see  Fig.  7).  Seen 
from  the  side,  however,  several  curves  come  into  view,  as 
shown  in  Fig.  10.  On  the  ventral  side,  in  the  cervical  and 


FIG.  7.   The  skeleton  entire 
15 


16 


THE  HUMAN  MECHANISM 


FIG.  8.    Sixth  thoracic 
vertebra 

Seen  from  above 


upper  thoracic  region,  the  curvature  is  slightly  convex ;  in 
the  thoracic  region  it  is  quite  concave ;  in  the  lumbar  region 
slightly  convex ;  and  in  the  sacral-coccygeal  region  again 

concave.  It  may  well  be  asked  how 
these  separate  vertebrae,  piled,  as  it 
were,  one  above  another,  maintain 
their  proper  relative  positions.  This 
is  partly  due  to  the  shape  of  the 
individual  vertebrae,  partly  to  the 
ligaments  (p.  17)  which  pass  from 
one  vertebra  to  another  and  limit 
the  movements  of  each,  and  partly 
to  the  action  of  muscles  which  are 
placed  upon  opposite  sides  of  the 
vertebrae  and  by  their  antagonistic 

action  hold  them  in  place.  The  action  of  muscles  and  liga- 
ments upon  the  bones  may  be  illustrated  by  two  blocks  of 
wood  held  together  by  two  rubber  bands  (w,  m',  Fig.  11) 
slightly  stretched ;  so  long  as  each 
pair  of  opposite  bands  pulls  with 
the  same  force,  the  blocks  are  kept 
iii  what  we  may  call  their  resting 
position.  Here  the  rubber  bands 
represent  two  of  the  antagonistic 
muscles,  which,  by  maintaining  a 
steady  and  equal  pull  on  the  oppo- 
site sides  of  the  vertebree,  keep 
them  in  place.  Should  one  pull 
harder  than  its  antagonist,  as  when 
a  muscle  contracts  (see  Chap.  IV), 
the  antagonist  will  be  stretched 
and  the  bones  become  inclined 
toward  one  another,  as  shown  in  right  portion  of  Fig.  11. 
This  principle  of  muscular  antagonism  is  quite  general  in 
the  maintenance  of  the  proper  relative  positions  of  bones  in 


FIG.  9.    Sixth  thoracic 
vertebra 

Seen  from  the  side 


STRUCTURE  OF  THE  HUMAN  MECHANISM       17 


Cervical 


Thoracic 
(or  dorsal) 


10 


the  body.  Almost  every  joint  is  the  theater  of  such  plays 
of  antagonistic  muscles,  which  serve  the  double  function  of 
keeping  the  bones  in  proper  position  with  regard  to  one  an- 
other and  of  producing  movement  at  the  joint,  the  amount 
of  this  movement  being  limited  by 
the  slack  but  inextensible  connective- 
tissue  ligaments  which  bind  the  bones 
together.  In  Fig.  11  both  the  short- 
ening of  the  muscle  and  the  slack- 
ness of  the  ligaments  are  purposely 
exaggerated,  in  order  to  represent 
more  clearly  the  functions  of  these 
tissues.  Ligaments  may  also  guide 
the  movement  of  bones  by  pre- 
venting motion  in  one  direction 
or  another. 

10.  The  ribs.    Each  rib   consists 
of   a  bony  and  a   cartilaginous  por- 
tion.   The  former  articulates  (that 
is,  forms  a  joint  with)  the  vertebral 
column,   while  the  latter  continues 
this    bony    portion    to    the    ventral 
median   breastbone,    to    which    it    is 
directly  joined.    The  ribs  form  the 
framework  for  the  thorax  and  may 
be    lifted    or    lowered    by    muscles 
which  connect  them  with  the  verte- 
bral column  and  other  parts  of  the 
skeleton  (see  Fig.  12). 

11.  The  skeleton  and  the  central 

nervous  system.  The  skull  consists  of  the  bones  of  the 
face  and  those  of  the  cranium,  the  latter  holding  the  brain. 
It  is  supported  on  the  spinal,  or  vertebral,  column,  whose 
ringlike  vertebrae  inclose  a  bony  canal  continuous  with  the 
cranial  cavity.  This  is  known  as  the  spinal,  or  vertebral. 


Lumbar 


Sacral 


FIG.  10.   The  vertebral 
umn 

Seen  from  the  side 


18 


THE  HUMAN  MECHANISM 


canal,  in  which  lies  the  spinal  cord1  —  the  continuation  of 
the  central  nervous  system  posterior  to  the  brain. 


FIG. 11 

Model  showing  the  action  of  muscles  on  two  vertebrae  and  of  the  ligaments  (I,  I') 

in  limiting  the  amount  of  movement.   The  contraction  of  the  muscle  m  stretches 

its  antagonist  m'.  The  amount  of  movement  is  greatly  exaggerated 

Nerves,  which  pass  through  small  openings  in  the  cranium 
and  between  the  vertebrae,  leave  the  brain  and  cord  and 
end  in  the  muscles,  skin,  glands, 
and  other  organs  of  the  body  (see 
Chap.  VII). 

12.  The  appendicular  skeleton. 
The  bones  of  the  arm,  leg,  hand, 
and  foot  may  readily  be  felt  and 
are  sufficiently  familiar.  We  may, 
however,  call  attention  to  the  simi- 
larity in  the  number  and  form  of 
the  bones  of  the  arms  and  legs,  a 

similarity  which  is  not  only  helpful 

FIG.  12.  Dorsal  view  of  ver- 

!The  terms  ft spinal  cord,"  "spinal  col-  tebrse  and  ribs 

limn,"   and  "spinal  canal"  are   sometimes     showing  some  of  the  muscles 
confused  by  beginners.    The  spinal  column        which  lift  or  raise  the  ribs 
is  the   entire   bony  framework  formed  by 

the  vertebrae  —  the  whole  backbone  ;  it  surrounds  the  spinal  canal,  which, 
in  turn,  contains  that  part  of  the  nervous  system  known  as  the  spinal  cord. 


STRUCTURE  OF  THE  HUMAN  MECHANISM       19 


Vertebral 
Canal 


in  mastering  their  names  and  arrangement  but  is  also  sug- 
gestive of  the  similarity  of  function  in  quadrupeds,  both 
limbs  in  these  animals  be- 
ing organs  of  locomotion.  /»@JTL ;.Jr .x x.^  Cranium 

ARM 

Humerus,  single  long  bone  of  the 
upper  arm. 

Radius  and  ulna,  two  nearly  par- 
allel bones  of  the  forearm. 

Eight  small  irregular  bones  of 
the  wrist. 

Five  parallel  bones  of  the  palm. 

f  Thumb,  two  bones. 
Bones  ot  I  „ ,,       ~ 

4.  Other  fingers, 

finSers[     three  bones.  Thorax 

LEG 

Femur,  single  long  bone  of  the 
thigh. 

Tibia  and  fibula,  two  nearly  par- 
allel bones  of  the  lower  leg. 

Seven  small  irregular  bones  of 
the  ankle  and  heel. 

Five  parallel  bones  of  the  instep. 

.  f  Great  toe,  two  bones. 
Bones  of     ~- , 

•I  Other  toes, 

[     three  bones. 

The   legs   are  attached  to     FIG.  13.   Median  dorsoventral  section 
the  vertebral  column  by  the 

large  hip  bones,  which  articulate  directly  and  immovably 
with  the.  sacrum 1 ;  but  the  humerus,  or  bone  of  the  upper 
arm,  articulates  on  each  side  with  one  of  a  pair  of  bones 
which  form  the  shoulder  girdle,  or  skeleton  of  the  shoulder 
region ;  this  pair  consists  of  the  collar  bone  (clavicle)  ven- 
trally  and  the  shoulder  blade  (scapula^  dorsally.  The  clavicle 
articulates  with  the  head  of  the  breastbone ;  otherwise  the 
shoulder  girdle,  with  the  arm  attached  to  it,  is  connected 
1  The  sacrum  and  the  two  hip  bones  together  form  the  pelvis. 


Sacrum 


Pelvis 


20 


THE  HUMAN  MECHANISM 


with  the  axial  skeleton  by  muscles  only.    A  wide  range  of 
movement  is  thus  secured  at  the  shoulder  joint. 

13.  Organs  of  digestion.  The  digestive  system  consists  es- 
sentially of  a  long  tube,  the  alimentary  canal,  passing  through 
the  body.1  Into  this  tube,  at  various  points,  ducts  from  a 
number  of  glands  pour  digestive  juices.  The  alimentary 
canal  begins  with  the  mouth  cavity 
and  its  familiar  organs,  the  teeth,  the 
tongue,  etc. ;  this  cavity  opens  pos- 
teriorly into  that  of  the  pharynx, 
into  which  also  opens  the  nasal 
cavity,  separated-  from  the  mouth 
only  by  the  palate  (see  Fig.  14). 

On  the  ventral  side  of  the  pharynx, 
just  beyond  the  root  of  the  tongue,  is 
the  slitlike  opening  of  the  windpipe 
(see  sect.  14) ;  posteriorly  the  pharynx 
is  continued  in  the  long  gullet,  or 
oesophagus,  a  tube  which  passes  down- 
ward through  the  neck  and  thorax 
FIG.  14.  Diagrammatic  me-  (within  the  mediastinum)  to  join  the 

dian  dorsoventral  section    stomach,  which  it  enters  immediately 
of    the    nasal    and   throat       ,,  .          ,  ,      ,       ,.      , 

after  passing  through  the  diaphragm. 

The  stomach  is  a  large  pouch  with 
contractile  walls  permitting  adapta- 
tion of  its  size  to  the  bulk  of  food 
it  may  contain.  Its  situation  is  shown 
in  Fig.  155,  which  also  shows  how  it 
opens  on  the  right  side  of  the  body  into  the  very  long, 
coiled  small  intestine.  The  coils  of  this  part  of  the  tube 
may  be  followed  for  from  twenty  to  twenty-four  feet,  to  the 
large  intestine,  into  one  side  of  which  it  opens.  The  large 
intestine,  or  colon,  consists  of  three  portions:  the  first  ascend- 
ing on  the  right  side  to  the  general  level  of  the  stomach,  the 
1  See  Fig.  155  for  the  general  arrangement  of  the  organs  of  digestion. 


passages 

C,  nasal  cavities;  M,  mouth 
cavity;  T,  tongue;  E,  epi- 
glottis; G,  glottis,  or  opening 
from  the  pharynx  into  the 
trachea;  U,  the  end  of  the 
soft  palate ;  0,  oesophagus 


STRUCTURE  OF  THE  HUMAN  MECHANISM      21 

second  passing  transversely  at  this  level  from  right  to  left, 
and  the  third  descending  on  the  left  side  to  the  rectum,  the 
posterior  terminal  portion  of  the  digestive  tube. 

Numerous  glands  pour  secretions  through  ducts  into  the 
digestive  tube,  the  more  important,  with  their  places  of  dis- 
charge, being  the  following :  salivary  glands  (see  Chap.  Ill) 
—  mouth  ;  liver  —  beginning  of  small  intestine ;  pancreas  — 
beginning  of  small  intestine  (see  Fig.  54).  Smaller  glands 
empty  into  the  stomach  and  intestines  at  numerous  places. 

14.  The  organs  of  respiration.    The  organs   of  respiration 
consist  of  the  right  and  left  lungs  (see  Fig.  5),  from  each 
of  which  a  single  bronchus  (pi.  bronchi)  leads  to  the  trachea 
(or  windpipe).   The  walls  of  the  trachea  and  bronchi  are  kept 
from  collapsing  by  successive  rings  of  cartilage.    Anteriorly 
the  trachea  opens  into  the  pharynx  through  the  larynx,  or 
voice  box,  the  cartilages  of  which  may  be  felt  in  the  throat 
at  the  root  of  the  tongue.    The  familiar  hoarseness  which 
accompanies  inflammatory  roughening  of  the  lining  of    the 
larynx  shows  how  important  is  this  organ  in  the  production 
of  the  voice.    The  respiratory  and  digestive  paths  cross  in 
the  pharynx,  the  former  reaching  the   exterior  through  the 
nose,  the  latter  through  the  mouth. 

15.  The  organs  of  circulation.    The  position  of  the  heart 
and  the  great  blood  vessels  in  the  thorax  has  been  described 
on  page  11.    The  heart  is  essentially  a  large  mass  of  muscle 
containing   a  cavity  which   is   divided   into  right  and   left 
halves,  wholly  separate  from  each  other.    The  cavity  on  each 
side  is  divided  into  that  of  the  large  ventricle,  with  very  thick 
walls,  and  that  of  the  much  smaller  auricle.   The  heart  is  thus 
composed  of  right  and  left  auricles  and  right  and  left  ven- 
tricles.   Valves  are  so  placed  in  the  heart  as  to  allow  blood 
to  flow  in  one  direction  only  (see  Fig.  69). 

The  arteries  are  tubes  which  carry  the  blood  to  the  tis- 
sues, and  from  each  side  of  the  heart  a  single  artery  takes 
its  origin  —  the  pulmonary  artery  from  the  right  ventricle, 


22  THE  HUMAN  MECHANISM 

and  the  aorta  from  the  left  ventricle.  The  pulmonary  artery 
supplies  the  lungs  with  blood,  while  all  other  organs  are 
supplied  by  the  aorta. 

The  veins  are  tubes  which  conduct  the  blood  from  the 
various  organs  to  the  heart.  Beginning  in  the  tissues  as 
microscopic  tubes,  they  unite  to  form  larger  and  larger  tubes 
as 'they  approach  the  heart;  those  visible  through  the  skin 
of  the  hand  may  be  regarded  as  of  medium  size ;  as  the 
union  goes  on,  the  size  of  the  vessels  increases  until  finally 
at  the  heart  there  are  only  two  great  veins  on  the  right  side 
(superior  vena  cava  and  inferior  vena  cavd)  and  four  on  the 
left  (pulmonary  veins).  The  venae  cavae  bring  blood  back 
from  those  portions  of  the  body  which  are  supplied  by  the 
aorta,  that  is  to  say,  from  all  parts  of  the  body  except  the 
lungs ;  the  pulmonary  veins  bring  blood  back  only  from 
the  lungs,  that  is  to  say,  from  the  organs  supplied  by  the 
pulmonary  arteries.  The  venae  cavae  empty  into  the  right 
auricle,  the  pulmonary  veins  into  the  left  auricle.  The  gen- 
eral arrangement  of  heart,  arteries,  and  veins  is  shown  in 
Fig.  15,  and  the  figures  in  Chapter  IX  (especially  70  and  71) 
should  also  be  consulted. 

The  blood  flows  in  the  following  circuit: 

Right  ventricle  to 

Pulmonary  artery  to 
Pulmonary  circulations  T 

.Lungs  to 

Pulmonary  veins  to 

Left  auricle  to 

Left  ventricle  to 

Aorta  and  its  branches  to 

All  organs  of  the  body  (except  the  lungs)  to 

Veins  which  unite  to  form  the  venae  cavae  to 

Right  auricle  to 

Right  ventricle 

Thus  the  blood  which  leaves  the  left  ventricle  flows  to 
the  different  organs  of  the  body  (except  the  lungs)  and 
returns  by  way  of  the  veins  to  the  right  side  of  the  heart; 


Systemic  circulation 


FIG.  15.   Diagram  of  the  circulation  of  blood 

K.A.,  right  auricle ;  L.A.,  left  auricle ;  R.V.,  right  ventricle ;  L.V.,  left  ventricle ; 
P.A.,  pulmonary  artery ;  A,  pulmonary  artery  and  vein  of  right  lung ;  B,  pulmo- 
nary artery  and  vein  of  left  lung;  (7,  carotid  artery  to  head,  showing  branch 
of  left  subclavian  artery ;  D,  portal  vein ;  E,  hepatic  vein ;  F,  hepatic  artery  ; 
G,  jugular  vein,  bringing  blood  from  head  and  neck 


24 


THE  HUMAN  MECHANISM 


thence  it  passes  through  the  lungs  and  again  to  the  left 
auricle  and  ventricle,  thus  completing  the  "  circulation." 
The  term  "  circulation,"  strictly  speaking,  is  applied  to  the 
entire  circuit  which  the  blood  must  traverse  before  it  returns 
again  to  the  point  from  which  it  started;  it  is  often  con- 
venient, however,  to  use  it  to  denote  the  course  from  the 
right  ventricle  to  the  left  auricle,  or  from  the  left  ventricle 
to  the  right  auricle;  in  this  case  we  speak  of  the  former  as 

the  pulmonary  and  of 
the  latter  as  the  sys- 
temic, or  aortic,  circu- 
lation. In  this  sense 
there  may  be  said  to 
be  a  "  double  "  circu- 
lation. 

The  veins  have 
thinner  walls  than  the 
corresponding  arter- 
ies, and  those  of  the 
systemic  circulation 
contain  purplish  or 
even  bluish  blood, 
while  the  arteries  of 
the  same  circulation 
contain  bright-scarlet 
blood.  The  bright 
color  of  the  arterial  blood  is  due  to  the  fact  that  it  contains 
more  oxygen.  The  change  from  purple  to  scarlet  occurs  in 
the  lungs,  and  the  reverse  change  in  the  organs  supplied 
by  branches  of  the  aorta.  Consequently  the  blood  of  the 
pulmonary  arteries  is  blue,  or  venous,  in  color  and  that  of  the 
pulmonary  vein  scarlet,  or  arterial. 

16.  The  course  and  branches  of  the  pulmonary  artery  and 
vein.  Soon  after  leaving  the  right  ventricle  the  pulmonary 
artery  divides  into  two  branches,  one  going  to  each  lung. 


FIG.  16.    A  network  of  capillaries,  with  the 
artery  a  and  vein  v  (highly  magnified) 


STRUCTURE  OF  THE  HUMAN  MECHANISM       25 


Each  of  these  further  divides  as  it  plunges  into  the  substance 
of  the  lung  alongside  the  bronchus.  The  course  of  the  four 
pulmonary  veins  may  be  similarly  traced  into  the  lungs,  from 
which  they  bring  the  blood  back  to  the  heart  (Fig.  15). 

17.  The  course  and  branches  of  the  aorta.  The  aorta 
passes  anteriorly  from  the  left  ventricle,  but  very  soon 
arches  dorsally  and  posteri- 
orly, forming  the  arch  of  the 
aorta  (Fig.  15);  the  general 
course  of  the  artery  can  be 
best  understood  from  the  fig- 
ures or  from  actual  dissec- 
tion. The  arch  of  the  aorta  is 
continued  in  the  large  dorsal 
aorta,  which  passes  posteriorly 
on  the  left  side  of  the  me- 
diastinum near  the  spine, 
through  the  diaphragm,  to  the 
lower  portion  of  the  abdomi- 
nal cavity,  where  it  divides 
into  two  large  arteries  which 
supply  blood  to  the  hips  and 
legs.  From  the  arch  of  the 
aorta  three  large  arteries  pass 
to  the  head,  neck,  shoulders, 
and  arms ;  from  the  thoracic 
dorsal  aorta  arise  a  number  of 
small  arteries  which  supply 
the  muscles  and  other  organs  FIG.  17.  The  general  arrangement  of 
of  the  thoracic  wall ;  immedi-  the  nervous  system  <dorsal  view> 
ately  after  passing  through  the  diaphragm  two  large  branches 
go  to  the  stomach,  spleen,  liver,  pancreas,  and  a  large  part 
of  the  small  intestine;  posterior  to  these  the  renal  arteries 
pass  right  and  left  to  the  kidneys,  and  still  further  down 
a  large  artery  supplies  the  lower  small  intestine  and  the 


26  THE  HUMAN  MECHANISM 

large  intestine.  The  supply  to  the  legs  has  already  been 
mentioned.  Other  small  arteries  arise  from  the  abdominal 
aorta  and  are  distributed  to  the  muscles 
and  skin  of  the  back.  The  arteries  to  the 
stomach  and  intestine  lie  in  the  mesen- 
tery (Fig.  163)  and  their  course  may 
.be  readily  traced  in  a  dissection. 

18.  The  course  and  branches  of  the 
venae  cavae.  The  blood  which  has  thus 
been  distributed  from  the  aorta  returns 
to  the  opposite  side  of  the  heart  through 
the  veins  which  ultimately  form  the  two 
vense  cavae.  In  general,  it  may  be  stated 
that  the  veins  of  those  organs  which  are 
anterior  to  the  diaphragm  form  the  su- 
perior vena  cava,  while  those  posterior 
to  the  diaphragm  form  the  inferior  vena 
cava.  The  larger  veins  usually  run  near 
and  approximately  parallel  to  the  larger 
arteries.  This  is  the  case  with  those 
from  the  arms  and  legs,  the  kidneys, 
and  the  muscles  of  the  trunk.  One  nota- 
ble and  very  important  exception,  how- 
ever, is  found  in  the  venous  supply  of 
the  stomach,  spleen,  and  intestines,  the 
veins  of  which  unite  to  form  a  single 
large  vein  (portal  vein)  which  passes 
to  the  liver,  where  it  breaks  up  into 
smaller  vessels;  the  blood  which  has 
thus  passed  through  the  liver  is  finally 
collected  in  the  hepatic  vein  and  poured 
by  this  into  the  inferior  vena  cava  just 
before  the  latter  passes  through  the  dia- 
phragm on  its  way  to  the  right  ventricle 
FIG.  18.  Nerve  trunks  »  J  £ 

of  the  right  arm          (Fig.  15;   see  also  Ing.  70). 


STRUCTURE  OF  THE  HUMAN   MECHANISM      27 

19.  The  capillaries.     The   blood   which   enters   an   organ 
through  the  arteries  passes  to  its  veins  through  a  system  of 
microscopic  tubes  (Fig.  16),  the  capillaries  (Latin  capilla,  "  a 
hair") ;   these  may  be  readily  seen  under  the  microscope  in 
the  web  of  a  frog's  foot.    From  the  foregoing  description  of 
the  course  of  the  circulation  it  will  be  observed  that  gen- 
erally the  blood  must  pass  through  one  set  of  capillaries  in 
going  from  the  aorta  to  the  vena3   cavse  or  from  the  pul- 
monary artery  to  the  pulmonary  vein ;  but  the  blood  which 
flows  through  the  capillaries  of  most  of  the  abdominal  organs 
(stomach,  intestines,  spleen)  must  pass  also  through  a  second 
set  of  capillaries,  namely,  those  of  the  liver,  before  it  can 
return  to  the  heart. 

20.  Organs  of  the  nervous   system.     The   skull   and   the 
spinal  column  (p.  18)  are  chiefly  occupied  by  the  brain  and 
the  spinal  cord,  respectively,  and  from  each  of  these  principal 
organs  of  the  nervous  system  branches  consisting  of  cords 
of  nervous  substance,   the   nerves,   pass   out   through   small 
holes  in  the  skull  or  spinal  column  and  are  distributed  to 
all  the  other  organs,  where  they  terminate  in  peculiar  struc- 
tures called  end  organs.    The  optic  nerve,  for  example,  ends 
in  the  retina,  the  auditory  nerve  in  the  inner  ear,  and  motor 
nerves  in   muscles  —  the   nerve   endings   in   these    different 
organs  differing  materially  in  structure  and  arrangement. 

Fig.  17  gives  some  idea  of  the  general  arrangement  of 
the  nervous  system.  The  nerves  to  the  shoulder,  arm,  and 
hand  will  be  seen  to  arise  from  the  cervical  region  of  the 
spinal  cord ;  those  for  the  trunk,  from  the  dorsal  and  lumbar 
regions ;  those  for  the  legs,  from  the  sacral  region.  The  head 
and  face  receive  nerves  from  the  posterior  portions  of  the 
brain.  The  dissection  of  the  arm  in  Fig.  18  shows  more 
accurately  the  main  nerve  trunks  to  that  region.  Further 
information  with  regard  to  the  structure  of  the  nervous 
system  will  be  given  in  Chapters  VII,  XIV,  and  XV. 


CHAPTER  III 

THE  FINER  STRUCTURE  OF  TWO  TYPICAL  ORGANS, 

GLANDS  AND  MUSCLES.    THE  CONNECTIVE  TISSUES. 

THE  LYMPHATIC  SYSTEM 

In  the  previous  chapter  we  have  examined  the  general 
construction  of  the  human  machine  as  regards  its  more 
conspicuous  parts  or  organs,  and  especially  their  location,— 
whether  internal  or  external,  dorsal  or  ventral,  anterior  or 
posterior,  on  the  right  or  on  the  left,  —  their  relations  to 
certain  important  cavities,  and  their  combination  to  consti- 
tute the  mechanism  which  we  call  the  human  body.  We 
must  now  push  our  examination  further  and  investigate 
the  finer  structure  of  some  of  the  more  important  parts  of 
the  machine.  For  this  purpose  we  may  select  two  typical 
organs,  a  gland  and  a  muscle,  the  one  unfamiliar,  by  name 
at  least,  to  most  people,  the  other  well  known  in  the  form 
of  steaks,  chops,  roast  beef,  and  other  meats. 

1.  What  is  a  gland?  A  gland  is  a  mass  of  tissue,  gen- 
erally softer  than  muscle  and  of  no  special  size  or  shape, 
though  often  rounded  or  egg-shaped.  The  gland  most 
easily  seen  is  the  milk  gland  or  udder  of  the  cow.  This 
is  a  large  mass  of  soft  tissues  devoted  to  manufactur- 
ing or  secreting  milk.  In  general,  glands  are  manufac- 
turing organs  for  the  preparation  of  saliva,  gastric  juice, 
bile,  tears,  sweat,  or  other  secretions.  Some  have  tubes, 
or  ducts,  through  which  their  secretions  are  carried  away; 
others  have  no  such  outlets  and  hence  are  known  as  duct- 
less glands.  Glands  vary  in  size  from  some  which  are 
microscopic  to  the  huge  liver,  which  is  the  largest  single 

28 


TYPICAL  STRUCTURE  OF  ORGANS 


29 


organ  in  the  human  body  (see  Fig.  2).  The  pancreas,  or 
"  sweetbread,"  of  the  calf  is  an  excellent  gland  for  the 
beginner  to  dissect  or  study. 

2.  A  typical  gland.    If  we  have  before  us  the  whole  or  a 
part  of  any  typical  gland,  we  find  that  we  are  dealing  with 
a  comparatively  soft  and  sometimes  even  pulpy  mass  held 
together  by  a  loose  mesh  or  network  of  harder,  tougher,  and 
more  or  less  fibrous  materials. 

A  pancreas  or  a  liver,  if  en- 
tire, shows  conspicuous  lobes, 
and  in  the  pancreas  these 
lobes  are  plainly  subdivided 
into  smaller  lobes,  or  lobules. 
In  favorable  specimens  tubes 
may  be  seen  connected  with 
the  gland ;  some  of  these  are 
blood  vessels  supplying  blood 
to  the  gland,  and  one  of  them 
is  a  duct  draining  away  from 
it  the  liquid  which  the  gland 
has  manufactured  or  secreted. 
After  a  preliminary  examina- 
tion of  this  sort  of  some  edible 
gland,  preferably  the  pancreas, 
we  may  pass  on  to  consider  in  greater  detail  one  of  our  own 
salivary  glands,  of  which  we  have  two  on  each  side  of  the 
head,  namely,  one  parotid  and  one  submaxillary  gland. 

3.  The  structure  of  the  submaxillary  gland.    The  two  sub- 
maxillary  glands  lie,  one  on  each  side  of  the  face,  embedded 
in  the  tissues  between  the  lower  jaw  and  the  upper  portion 
of  the  neck.    From  each  gland  a  duct  passes  forward  in  the 
tissues  forming  the  floor  of  the  mouth,  into  which  it  opens 
by  one  of  the  small  eminences,  or  papillce,  under  the  tongue. 
Through    this   duct    the   gland    pours   into   the   mouth    its 
secretion,  saliva. 


FIG.  19.    Diagram  of  submaxillary 
gland 

D,  its  duct ;  N,  its  nerve ;  A,  its  artery ; 
V,  its  vein ;  T,  tongue 


30 


THE  HUMAN  MECHANISM 


If  the  gland  were  to  be  cut  in  two  in  any  direction  with 
a  sharp  knife,  we  should  see  at  once  that  it  is  composed  of 
separate  parts,  or  lobes,  and  that  these  lobes  are  still  further 
divided  into  smaller  portions,  or  lobules.  The  lobules  and 
lobes  are  bound  together  with  a  rather  loose  connective  tissue 
which  is  continuous  with  a  somewhat  denser  layer  surround- 
ing the  gland  and  forming  its  capsule ;  the  connective  tissue 

between  the  lobes  forms 
the  primary  septa  (sing., 
septum)  and  that  between 
the  lobules  the  secondary 
septa.  The  relation  of 
these  structures  is  shown 
in  Fig.  20.  With  the  aid  of 
the  microscope  we  find  that 
each  lobule  is  still  further 
divided  by  connective  tis- 
sue into  flask-shaped  struc- 
tures, or  alveoli  (sing., 
alveolus)  :  in  these  the  se- 
cretion, saliva,  is  manufac- 
tured and  from  them  it  is 
discharged  into  the  duct  of 
which  the  alveoli  are  the 
blind  ends  (Fig.  21). 

The  whole  gland  may  be 
compared  to  a  large  bunch 
of  grapes ;  the  main  tubular 
duct  of  the  gland  branches  (in  the  septa  of  connective  tissue) 
very  much  as  the  stem  of  the  bunch  of  grapes  branches ;  and 
just  as  the  branches  and  subbranches  of  the  stem  lead,  when 
followed  up,  to  the  grapes  themselves,  so  the  branches  of  the 
duct  lead  to  the  alveoli  of  the  gland.  If  now  we  pack  the 
bunch  of  grapes  in  a  small  basket  of  sawdust  or  cork  waste, 
as  Malaga  grapes  are  packed,  so  that  the  sawdust  fills  up 


FIG.  20.    Diagram  of  a  cross  section  of 
a  gland 

Showing  its  division  by  primary  septa  (S) 
into  lobes  and  by  secondary  septa  (s)  into 
lobules;  also  the  origin  of  the  larger 
branches  of  the  duct  (D)  in  the  lobes  and 
lobules.  The  beginnings  of  the  duct  are 
shown  in  Figs.  21  and  22 


TYPICAL  STRUCTURE  OF  ORGANS 


31 


loosely  the  spaces  between  the  individual  grapes  and  the 
branches  of  the  stem,  we  shall  have  something  with  which 
to  compare  the  arrangement  of  the  connective  tissue  in 
relation  to  the  rest  of  the  gland  —  the  sawdust  standing 
for  the  connective  tissue  in  which  the  ducts  and  alveoli 
are  embedded,  and  the  basket  for  the  capsule. 


Duct 


Alveoli 


Secreting  Cells 
Capillary  Network 


FIG.  21.    The  origin  of  the  duct  of  a  gland  in  alveoli,  together  with  the 
connective  tissue  and  blood  vessels 

4.  Minute  structure  of  ducts  and  alveoli.  The  alveoli  are 
not,  however,  empty  shells  like  glass  flasks  nor  solid  masses 
like  grapes,  but  rather  hollow  bags  lined  with  a  layer  of 
thickish,  closely  set  cells,  all  much  alike.  Each  of  these  cells 
consists  of  two  portions  —  a  small  central  body,  the  nucleus, 
and  a  larger  surrounding  mass,  the  cytoplasm.  All  organs 
of  the'  body  are  composed  of  cells,  differing  in  different 
organs  or  in  different  parts  of  the  same  organ  (as  in  the 
duct  and  the  alveolus  of  the  gland),  but  all  consisting  of  two 
never-failing  parts  —  nucleus  and  cytoplasm. 


32  THE  HUMAN  MECHANISM 

The  muscle  and  the  gland  consist  of  cells,  just  as  all  the 
branches  of  the  military  service  —  the  infantry,  the  cavalry, 
the  artillery,  the  engineers,  etc.  —  consist  of  men.  The  cell 
is  the  anatomical  or  fundamental  unit  of  these  organs,  as  the 
soldier  is  the  fundamental  or  anatomical  unit  of  the  army; 
in  both  cases  the  anatomical  units,  differing  in  equipment 
and  training,  perform  different  kinds  of  work,  yet  have  the 


FIG.  22.    Section  of  a  portion  of  a  salivary  gland  (magnified  500  diameters) 

After  Kcelliker 

The  duct  d  divides  into  the  two  branches  d'  and  d" ,  one  of  which  ends  in  the 

alveoli,  a,  a.   Neighboring  alveoli,  a',  a',  whose  ducts  are  not  in  the  plane  of  the 

section,  are  also  shown.    In  some  cells  the  section  does  not  include  the  nucleus, 

which  would  be  in  the  preceding  or  the  succeeding  section 

same  essential  structure ;  and  the  cells  are  combined  into 
brigades,  divisions,  or  corps,  as  tissues  and  organs ;  they  make 
of  the  body  an  army  organized  to  fight  its  way  through  the 
vicissitudes  and  against  the  obstacles  of  life. 

5.  The  structure  of  the  biceps  muscle.  The  biceps  muscle 
is  familiar  as  the  mass  of  flesh  lying  on  the  front  of  the 
upper  arm  and  bulging  somewhat  when  the  arm  is  bent  at 
the  elbow,  especially  when  one  "feels  his  muscle"  or  when 
a  weight  is  being  lifted  by  the  hand.  Figure  23  shows  this 
muscle  with  the  bones  to  which  it  is  attached.  It  consists  of 


TYPICAL  STRUCTURE  OF  ORGANS 


33 


two  portions:  a  central,  thick,  red  part,  known  as  the  belly, 
soft  when  the  muscle  is  at  rest,  hard  when  it  is  contracted ; 
and  cordlike  strings,  or  tendons,  two  at  the  upper  end  and  one 
at  the  lower,  by  means  of  which  the  muscle  is  attached  to  two 
bones  of  the  shoulder  girdle  and  to  one  of  the  forearm.  When 
the  belly  of  the  muscle  shortens,  the  points  a  and  b  are  brought 
closer  together  and  the  arm  is  bent,  or  flexed,  at  the  elbow. 
This  drawing  together,  or  contraction,  is  the  special  work,  or 
function,  of  muscles  in  general. 

Everyone  has  seen  the  cross 
section  of  a  muscle  in  a  raw 
beefsteak.  This  shows  that  the 
muscle  as  a  whole  is  surrounded 
by  a  sheath  of  connective  tissue 
which  contains  more  or  less  fat ; 
septa  pass  inwards,  dividing  the 
muscle  into  lesser  red  masses 
known  as  fasciculi,  or  bundles, 
and  these  are  further  subdivided 
into  secondary  fasciculi  by  sec- 
ondary septa,  very  much  as  the 
gland  is  subdivided  into  lobules. 

A  longitudinal  section  shows 
that  the  fasciculi  run  from  ten- 
don to  tendon,  and  microscopic 
examination  proves  that  the  general  connective  tissues  of  the 
belly  of  the  muscle  are  continuous  with  that  of  the  tendon. 
The  tendon  itself  is  a  peculiarly  strong  and  inextensible 
variety  of  connective  tissue  consisting  chiefly  of  parallel 
fibers  which  are  specially  fitted  to  transmit  to  the  bone  the 
pull  of  the  belly  of  the  muscle. 

6.  The  muscle  fibers.  Examination  of  the  structure  of 
one  of  the  finer  fasciculi  in  the  belly  of  the  muscle  shows 
that  it  is  composed  of  threads,  or  fibers,  which  at  first  sight 
differ  greatly  from  the  secreting  cells  of  the  gland.  These 


FIG.  23.    The  biceps  muscle  of 
the  arm 

The  resting  condition  is  shown  by 
the  solid  lines,  the  contracting  con- 
dition by  the  dotted  lines 


34 


THE  HUMAN  MECHANISM 


are  the  muscle  fibers.    They  are  1  to  1J  inches  in  length  and 


FIG.  24.     Tendon 
(highly  magnified) 

Showing  the  fiber 
bundles    separated 


from  TrrW  to  ^l"^  of  an  inch  in  thickness,  thus  being  from 
250  to  2500  times .  as  long  as  wide,  and 
comparable  in  shape  to  a  long  leather  shoe- 
string rather  than  to  a  sausage.  Each  fascic- 
ulus contains  hundreds  or  even  thousands 
of  fibers.  The  fibers  always  run  lengthwise 
of  the  fasciculus,  but,  as  a  usual  thing,  do 
not  extend  its  entire  length,  as  obviously 
follows  from  the  fact  that  a  single  fasciculus 
of  the  biceps  is  several  inches  in  length. 
The  fibers  are  inclosed  in  a  very  thin  trans- 
parent membrane,  the  sarcolemma,  and  are 
bound  into  bundles  (or  fasciculi)  by  the 
same  fine  connective  tissues  seen  between 
the  alveoli  of  a  gland.  To  the  end  of 
the  sarcolemma  are  attached  fine  fibers  of 
connective  tissue  which  pass  into  the  tendon 
(Fig.  25).  Indeed,  the  fibers  of  the  tendon  are 
the  collected  fibers  from  the  sarcolemmas  of  all 
the  muscle  fibers.  For  this  reason  the  part 
of  the  muscle  near  the  tendon  is  "  tough  meat," 
while  that  in  the  belly  of  the  muscle  is  tender, 
owing  to  the  smaller  number  of  connective- 
tissue  fibers. 

7.  The  muscle  fiber  a  cell.  The  muscle  fiber 
at  first  sight  does  not  seem  like  the  typical  cell 
already  described,  with  nucleus  and  cytoplasm  ; 
for  when  examined  in  the  fresh  condition  the 
only  obvious  points  of  structure  seen  in  it  are  showing  the  at- 
striking  cross  striations  consisting  of  alternate  tachment  of  the 
dark  and  light  bands.  It  has  been  shown,  how-  t£g  sarco^mnm 
ever,  by  ingenious  and  careful  study  that  the 
cross  striations  are  optical  appearances  produced  by  the 
peculiar  shape  of  extremely  minute  longitudinal  rods  in  tho 


FIG.  25.     One 
end  of  a  mus- 
cle fiber 


TYPICAL  STRUCTURE  OF  ORGANS 


35 


cytoplasm  of  tlie  muscle  fiber  and  that,  immediately  under 
the  sarcolemma,  xdiere  are  numerous  characteristic  nuclei 
which  are  easily  brbught  into  view  by  suitable  treatment. 
Briefly,  then,  the  muscle  fiber  is  a  cell  with  many  nuclei,  in 
whose  cytoplasm  are  found  peculiar  structures,  the  myofibrils ; 
upon  superficial  examination  these  myofibrils  not  only  obscure 
the  nuclei  but  give  to  the  whole  fiber  the 
appearance  of  cross  striation. 

8.  How  far  is  the  structure  of  glands 
and  muscles  typical  of  all  organs?  Both 
the  gland  and  the  muscle  are  thus  com- 
posed of  cells.  Although  differing  con- 
siderably in  the  two  organs,  these  cells 
possess  certain  general  and  fundamental 
features  in  common,  for  each  one  contains 
a  nucleus  (or  nuclei)  and  surrounding 
cytoplasm.  Is  the  same  thing  true  of  all 
other  organs  ?  The  muscle  and  the  gland 
are  examples  of  organs  which  do  active 
work,  but  some  other  organs  perform 
purely  passive  functions.  Such  are  the 
bones,  which  do  no  work  themselves,  but 
upon  which  the  work  of  mechanical  motion 
is  done  by  the  muscles ;  the  tendons,  which 
transmit  the  pull  of  muscles ;  the  ligaments, 
which  limit  and  sometimes  guide  the  mo- 
tion  of  bones;  and  the  connective  tissues, 
which  bind  together  other  parts  of  the  body.  None  of  these 
is  a  working  organ  in  the  sense  that  a  muscle  or  a  gland  is 
a  working  organ,  and  we  are  not  surprised  to  find  that  their 
structure  departs  from  that  of  the  muscle  and  gland  in  that, 
while  nucleated  cells  are  present  in  all  of  them,  the  great  mass 
of  the  organ  is  composed  of  lifeless  matter  between  the  cells.  In 
a  tendon  this  consists  of  very  strong  parallel  fibers  (Fig.  24) ; 
a  ligament  shows  much  the  same  structure ;  a  bone  consists 


FIG.  26.    Part  of  a 
muscle  fiber 

Specially  prepared  to 
bring  out  th 


36 


THE  HUMAN  MECHANISM 


chiefly  of  lifeless  material  containing  large  amounts  of  mineral 
matter,  with  cells  lying  here  and  there  in  spaces  which  com- 
municate with  one  another  by  means  of  minute  channels.  The 
connective  tissues,  like  that  which  binds  the  skin  to  the  un- 
derlying muscles  or  that  which  forms  the  sheath  and  septa  of 
glands  and  muscles,  consist  essentially  of  lifeless  fibers  run- 
ning in  all  directions  and  thus  ready  to  limit  the  extent  of  any 
pull  tending  to  separate  unduly  the  adjacent  organs.  To  organs 
and  tissues  of  this  kind  we  may  give  the  name  of  supporting 

organs  and  tissues, 
and  they  form  al- 
most the  sole  excep- 
tion to  the  general 
rule  that  the  essen- 
tial part  of  a  tissue 
consists  of  its  cells. 
The  latter  state- 
ment is  true  of  all 
organs  and  tissues 
which  do  work  — 
the  active  organs  of 
the  body.  In  the 
case  of  the  support- 
ing tissues  the  cells 
which  they  contain 
are  the  fundamental  units  of  the  organ,  since  they  make  the 
intercellular  lifeless  substance ;  but  the  part  which  the  organ 
plays  in  the  work  of  the  body  as  a  whole  is  performed 
by  the  lifeless  substance  (fibers,  etc.)  which  the  cells  have 
manufactured  and  keep  in  repair. 

9.  The  blood  vessels  are  closed  tubes  in  connective  tissue. 
The  arrangement  of  connective  tissues  is  fundamentally  the 
same  in  gland  and  muscle.  These  tissues  serve  the  obvious 
purpose  of  binding  the  anatomical  units  into  organs,  but 
they  also  perform  other  functions  equally  important. 


FIG.  27.    Longitudinal   (A)   and  transverse   (B) 
sections  of  bone 

Showing  the  branching  and  communicating  canals  — 
in  which  are  blood  vessels  and  nerves  —  surrounded 
by  the  lifeless  bone  substance.  In  this  are  spaces 
connected  with  one  another  by  very  minute  channels. 
Each  of  these  spaces  contains  a  living  cell,  shown  in  O 


TYPICAL  STRUCTURE  OF  ORGANS 


37 


We  have  seen  (Chap.  II,  sect.  19)  that  each  organ  receives 
blood  through  one  or  more  arteries,  and  that  this  blood  flows 
away  from  the  organ  through  one  or  more  veins.  If  a  colored 
fluid  mass  which  would  afterwards  set  (for  example,  a  warm 
solution  of  gelatin  colored  with  carmine)  had  been  forced 
into  the  arteries  before  we  began  our  examination,  we  should 
find  that  this  mass  would  every- 
where be  confined  in  a  system  of 
closed  tubes  which  merely  lie  in 
the  connective  tissue.  The  artery 
entering  the  muscle  branches  into 
smaller  and  smaller  arteries  in  the 
general  sheath  of  the  organ,  or  in 
its  branches,  the  septa;  from  these 
finer  arteries  an  exceedingly  rich 
network  of  small  thin-walled  tubes 
is  given  off  to  the  finest  connective 
tissue  which  surrounds  the  cells 
themselves ;  these  tubes  are  the 
capillaries.  They  ultimately  unite 
to  form  the  larger  veins,  which  can 
be  traced  in  the  septa  to  those  veins 
which  gross  dissection  reveals  as 
leaving  the  organ  (see  Fig.  19). 
Through  these  tubes — arteries,  capil- 
laries, and  veins  —  the  blood  flows; 
and  it  is  important  for  us  to  under- 


FIG.  28.  Three  muscle  fibers 
and  an  artery  breaking  up 
into  capillaries  between  them 


stand  that  it  is  everywhere  confined  to  them  in  its  passage 
through  the  organs ;  nowhere  does  it  come  into  direct  contact  with 
the  living  cells  (save  those  lining  the  vessels).  Whatever  ex- 
change of  matter  or  energy  takes  place  between  the  blood  and 
the  living  cells  must  be  through  the  walls  of  the  blood  vessels.1 

1  The  term  "  blood  vessel "  is  sometimes  confusing  to  the  beginner,  since 
it  suggests  a  utensil  for  holding  liquids.  In  anatomy  "vessel"  te  a  name 
for  tubes,  ducts,  or  canals  through  which  blood  or  lymph  flows. 


38 


THE  HUMAN  MECHANISM 


These  walls  are  relatively  thick  in  the  arteries,  usually  some- 
what thinner  in  the  veins;  in  the  capillaries,  however,  they 
are  very  thin,  and  it  is  through  these  thin  capillary  walls 
that  all  interchanges  of  matter  take  place.  That  the  con- 
nective tissue  surrounding  the  capillaries  bears  an  important 
relation  to  the  circulation  we  shall  now  see. 

10.  The  lymph  spaces  of  the  connective  tissue ;  the  lymph. 
Careful  examination  shows  that  the   fine  connective  tissue 

within  which  the  capillaries 
are  embedded  is  not  a  solid  or 
continuous  mass,  but  rather 
a  mass  or  mesh  of  extremely 
fine  fibers  or  bundles  of 
fibers,  with  here  and  there 
connective-tissue  cells  which 
keep  the  fibers  in  repair. 
The  connective  tissue,  there- 
fore, is  everywhere  channeled 
by  irregular  spaces  running 
between  the  fibers  and  other 
structures ;  these  spaces  com- 
municate freely  with  each 
other  and  contain  a  colorless 
liquid  known  as  lymph ;  the 
spaces  of  the  connective  tis- 
sue may  thus  be  conveniently 
described  as  lymph  spaces. 
They  serve  as  communicating 
channels  between  the  cells  and  the  walls  of  the  capillaries. 

11.  Origin  of  the  lymph.    The  lymph  which  the  spaces 
contain  is  derived  partly  from  water  and  soluble  food  mate- 
rials which  have  passed  through  the  capillary  walls  from  the 
blood  and  partly  from  material  produced  by  the  neighboring 
cells  (see  the  next  chapter);    on  the  other  hand,  the  cells 
absorb    from   the   lymph   substances   which    the    latter   has 


FIG.  29.  Superficial  and  some  deeper 
lymphatics  of  the  hand 


TYPICAL  STRUCTURE  OF  ORGANS 


39 


received  from  the  blood,  while  the 
the  lymph  substances  discharged 
from  the  cells.  The  lymph  thus 
becomes  the  means  of  communi- 
cation, the  middleman,  between 
the  living  cells  of  the  organs 
and  the  nourishing  blood,  and 
forms  the  immediate  environment 
of  the  cells  themselves.  In  other 
words,  the  cells  of  muscles,  glands, 
and  other  organs  live  in  lymph,  just 
as  the  human  body  as  a  whole 
lives  in  air,  or  a  fish  in  water. 

12.  The  lymphatics.  Besides 
the  veins,  which  convey  blood 
away  from  an  organ,  a  second 
system  of  tubes  or  vessels  passes 
out  through  the  capsule.  These 
tubes  arise  in  the  lymph  spaces 
of  the  connective  tissue  and 
unite  with  similar  tubes  from 
other  regions  to  form  larger  and  . 
larger  trunks,  known  as  lym- 
phatics, which  ultimately  form 
one  or  two  great  trunks  and 
open  into  the  great  veins  near 
the  heart  (see  Fig.  30).  Through 
these  direct  outlets  the  surplus 
lymph  of  the  organ  flows  in  a 
varying  but  for  the  most  part 
continuous  stream.  This  flow 
of  lymph  away  from  an  organ 
is  of  the  very  greatest  impor- 
tance in  maintaining  the  normal 
environment  of  the  cells. 


blood,  in  turn,  takes  from 


FIG.  30.  The  two  main  lymphatic 

trunks    (in    white),    with    their 

openings   into   the   great   veins 

near  the  heart 

The  larger  of  these  trunks  —  that 
on  the  left  side  and  known  as 
the  thoracic  duct  —  returns  all  the 
lymph  except  that  from  the  right 
side  of  the  head  and  neck  and  the 
right  arm  and  shoulder  region 


40 


THE  HUMAN  MECHANISM 


13.  Function  of  the  lymph  flow  from  an  organ.  It  is  clear 
from  inspection  of  Fig.  31  that  there  is  a  steady  flow  of 
liquid  from  the  capillaries,  through  the  lymph  spaces  of  the 
connective  tissue,  over  the  surfaces  of  the  living  cells  or  of 
any  intervening  capillaries,  to  the  lymphatics.  The  cell  is 
thus  bathed  not  by  a  stagnant  medium  but  by  one  which 
is  in  gentle  movement  —  one  which  brings  to  all  parts  of  its 


FIG.  31.    Diagram  of  the  relation  of  the  cells  of  an  organ  to  its  blood 

vessels,  lymphatics,  and  connective  tissue 

A,  artery;   V,  vein  ;  L,  lymphatic 

surface  the  food  which  it  needs  and  immediately  carries 
away  from  all  parts  of  its  surface  to  the  adjacent  capillaries 
the  products  of  its  activity.  By  providing  this  outlet  from 
the  lymph  spaces  the  lymphatics  render  possible  the  move- 
ment of  lymph  within  the  organ  itself,  whereby  material  is 
readily  transferred  from  the  cell  to  the  capillaries  and  from 
the  capillaries  to  the  cell. 

14.  Distribution  of  nerves  to  muscles  and  glands.    The  dis- 
tribution of  nerves  resembles  that  of  the  arteries,  the  larger 


TYPICAL  STRUCTURE  OF  ORGANS  41 

nerve  trunks  being  found  in  the  septa,  and  their  fine  ultimate 
branches  being  distributed  by  way  of  the  connective  tissue 
which  surrounds  the  cells,  in  whose  neighborhood  or  even 
within  whose  substance  they  finally  end.  As  we  shall  see  in 
subsequent  chapters,  it  is  the  function  of  the  nerves  to  arouse 
the  gland  cells  or  muscle  fibers  or  other  cells  to  activity. 

15.  Summary.  Disregarding  for  the  moment  those  pecu- 
liarities of  arrangement,  shape,  and  structure  of  the  cells 
which  are  connected  with  the  special  work  of  each  organ 
(for  example,  the  arrangement  of  gland  cells  to  form  a  blind 
tube  or  of  the  connective  tissue  and  fibers  of  muscle  so  as 
to  exert  a  pull  on  a  bone),  we  may  say  that  the  typical 
structure  of  an  organ  would  be  represented  in  Fig.  31.  The 
whole  is  surrounded  by  a  capsule,  receives  a  blood  supply 
through  a  system  of  closed  tubes,  and  contains  the  special 
cells '  upon  whose  activity  its  characteristic  work  depends. 
These  cells  are  held  together  by  a  fine  connective  tissue 
whose  numerous  and  freely  communicating  spaces  contain 
a  fluid,  the  lymph,  which  is  free  to  flow  out  through  a 
second  system  of  tubes,  the  lymphatics.  Nerves  from  the 
brain  or  spinal  cord  are  also  distributed  in  the  connective 
tissue  to  the  cells  of  the  organ. 

Before  concluding  this  description  of  the  finer  structure 
of  organs,  a  word  may  be  added  with  regard  to  the  physical 
nature  of  the  cell  substance.  In  its  literal  meaning  the  word 
"cell"  is  a  misnomer,  since  it  suggests  a  hollow  space  inclosed 
by  solid  partitions  or  walls.  Plant  cells  do,  in  fact,  usually 
have  such  walls  around  their  cytoplasm  (Chap.  VIII),  and 
this  cytoplasm  frequently  contains  spaces  (vacuoles)  filled 
with  a  solution  of  salts,  sugar,  and  other  dissolved  material ; 
but  neither  the  cell  wall  nor  vacuoles  are  of  universal  occur- 
rence, each  being  rarely  found  in  the  animal  cell,  and  often 
absent  even  in  the  plant  cell.  Fifty  years  of  thorough  investi- 
gation has  reduced  the  number  of  essential  cell  constituents 
to  the  cytoplasm  and  the  nucleus,  the  ultimate  structure  of 


42  THE  HUMAN  MECHANISM 

which  is  far  from  being  completely  understood.  It  would 
seem  that  the  cytoplasm  is  a  mixture  of  a  number  of  mate- 
rials which  differ  in  chemical  composition  and  in  physical 
properties.  Some  are  dissolved  in  water,  making  viscous 
solutions  comparable  to  the  white  of  egg  or  to  thick  or  thin 
jellies.  Others  are  known  as  lipins,  or  lipoids  (from  the  Greek 
lipos,  "a  fat"),  because  they  resemble  fats  or  oils  in  physical 
characters  and  to  some  extent  in  chemical  structure;  they 
do  not  mix,  or  mix  only  imperfectly,  with  the  viscous  aqueous 
(that  is,  watery)  solutions,  but  spread  over  the  outer  surface 
of  the  cell,  forming  a  membrane,  and  probably  also  penetrate 
into  the  cytoplasm  somewhat  as  the  connective-tissue  septa 
of  the  gland  penetrate  the  gland.  These  lipoid  membranes 
would  thus  separate  the  viscous  aqueous  solutions  of  the 
cytoplasm  into  separate  masses,  much  as  the  gland  is  divided 
by  its  septa  into  lobes  and  lobules.  The  lipins  are  supposed, 
among  other  functions,  to  control  the  passage  of  material  into 
and  out  of  the  cell.  The  cytoplasm  also  frequently  contains 
granules,  one  kind  of  which  we  have  already  seen  in  the 
zymogen  granules  of  the  gland  cells. 

The  nucleus,  on  the  other  hand,  is  known  to  contain  cer- 
tain other  compounds  peculiar  to  itself.  Some  of  these  at 
times  are  probably  in  an  almost  solid  state  and  appear  as 
denser  material  within  the  membrane  which  usually  bounds 
the  nucleus;  at  other  times  they  undergo  solution,  doubt- 
less as  the  result  of  chemical  changes  taking  place  within 
them.  There  are  many  strong  reasons  for  thinking  that  the 
nucleus  bears  an  important  relation  to  the  oxidations  of 
the  cell. 


CHAPTER  IV 
THE  OKGANS  AND  CELLS  OF  THE  BODY  AT  WOEK 

The  understanding  of  a  mechanism  involves  more  than  a 
knowledge  of  its  structure;  we  must  study  the  mechanism 
at  work,  and  the  human  mechanism,  which  we  are  studying, 
may  be  regarded  as  a  factory  in  which  work  is  done. 

The  work  of  some  manufacturing  establishments  consists 
in  separating  useful  constituents  of  the  raw  material  from 
useless  constituents,  as  where  kerosene  is  refined  from  crude 
petroleum;  that  of  others  consists  in  producing  chemical 
changes  in  the  raw  material,  as  where  soap  is  made  from  fat 
or  oil;  while  that  of  a  third  class  consists  in  the  application 
of  power  by  machinery,  as  where  lumber  is  sawed,  planed, 
turned,  or  molded  into  the  material  of  which  houses  are 
constructed.  The  work  of  a  factory,  in  other  words,  is  either 
a  process  of  refinement  or  the  production  of  a  new  substance  or 
the  application  of  power. 

The  human  body  is  a  factory  which  presents  in  its  activities 
examples  of  all  three  of  these  processes.  A  large  part  of 
digestion  is  a  process  of  food  refinement ;  out  of  the  food  we 
eat  the  very  substance  of  the  body  itself  is  formed;  while 
all  muscular  work,  including  the  beat  of  the  heart,  consists 
in  the  application  of  power  to  accomplish  useful  ends.  This 
work  is  done  chiefly  by  the  two  kinds  of  organs  whose  struc- 
ture we  have  just  studied,  namely  glands  and  muscles ;  and 
just  as  their  structure  presents  a  fundamental  similarity  of 
plan,  so  there  is  a  fundamental  similarity  in  the  nature  of 
their  activities.  This  can  best  be  made  clear  by  a  somewhat 
detailed  study  of  each  organ  at  work. 

43 


44  THE  HUMAN  MECHANISM 

1.  Physiology  of  the  salivary  glands;  working  glands  and 
resting  glands.    The  function  of  the  salivary  glands  is  the 
secretion  or  manufacture  of  saliva  for  use  in  the  mouth,  and 
one  of  the  first  things  we  notice  about  this  act  of  secretion 
is  that  it  is  not  constant  but  intermittent.    Most  organs  have 
periods  of  activity,  or  work,  followed  by  periods  of  inactivity, 
or  rest,  and  these  glands   are  no   exception.     Physiologists 
frequently  speak  of  "  working  glands  "  and  "  resting  glands." 
We  all  know  that  our  own  salivary  glands  work  more  effec- 
tively at  some  times  than  at  others.    The  mouth  "  waters " 
at  the  sight  of  food;  when  we  are  in  the  dentist's  chair  the 
flow  of  saliva  often  seems  excessive,  and  at  other  times  our 
mouths  are  "  parched  "  or  "  dry." 

2.  The  chemical  composition  of  saliva.    The  saliva  is  some- 
times thin  and  flows  readily,  while  at  other  times  it  is  thick 
and  viscous,  or  glairy.    This  difference  is  caused  by  the  fact 
that  the  amount  of  water  in  it  varies  under  different  con- 
ditions.   At  all  times,  however,  it  is  a  fluid  which  consists 
of  water  containing  certain  solids  in  solution.    The  amount 
of  these  solids  varies  from  five  to  ten  parts  in  a  thousand  of 
saliva,  and  they  consist  chiefly  of  three  groups  of  compounds. 
The  first  is  mucin,  familiar  to  us  as  the  chief  constituent  of 
the  phlegm  or  mucus  discharged  from  the  nose  and  throat, 
and  giving  to  the  fluid  its  viscous  character;   the  second 
group  consists  of  substances  known  as  enzymes,  those  in  the 
saliva  having  the  power  of  changing  starch  to  sugar;  these 
we  shall  study  in  detail  in  the  chapters  on  digestion;   the 
third  group  consists  of  mineral  or  inorganic  salts,  of  which 
ordinary  table  salt,  or  sodium  chloride,  is  the  most  important. 
As  we  shall  see,  the  salts  and  water  are  derived  directly  from 
the  blood,  while  the  mucin  and  enzymes  are  manufactured 
by  the  gland. 

3.  Blood  supply  of  the  working  gland.  Whenever  a  gland  is 
actively  working  there  is  an  increased  flow  of  blood  through 
it.    For  this  reason  the  resting  gland  is  slightly  pink,  while 


THE  WORK  OF  ORGANS  AND  CELLS  45 

the  working  gland  becomes  distinctly  red.  Since  the  secretion 
of  saliva  requires  water  and  this  can  be  obtained  only  from 
the  blood,  it  is  easy  to  see  why  an  abundant  blood  supply  is 
essential  to  activity.  Other  constituents  of  the  saliva,  such 
as  the  inorganic  salts,  likewise  come  directly  from  the  blood. 

4.  The  relation  of  nerves  to  gland  work ;  irritability.  Nerves 
pass,  as  we  have  seen  (p.  29),  from  the  central  nervous  sys- 
tem to  the  salivary  glands.  These  nerves  are  essentially 
bundles  of  nerve  fibers  which  are  distributed  from  the  brain 
and  spinal  cord  to  the  neighborhood  of  the  gland  cells. 
Such  fibers  are  the  means  of  conveying  to  the  gland  an 
influence,  called  a  nervous  impulse,  and  nervous  impulses 
cause  the  gland  to  secrete.  It  is  also  a  fact  that  when  these 
nerves  are  cut  or  injured  in  any  way,  so  that  the  gland  is 
no  longer  in  nervous  connection  with  the  brain  and  spinal 
cord,  saliva  is  not  secreted,  even  when  food  is  placed  in  the 
mouth.  Evidently  the  activity  of  the  gland  is  normally 
aroused  by  nervous  impulses  from  the  brain  and  spinal  cord, 
just  as  the  activity  of  a  receiving  instrument  in  a  telegraph 
office  is  aroused  by  the  electric  current  which  comes  to  it 
over  a  wire,  or  as  a  mine  is  exploded  by  the  same  means. 
The  gland  then  stands  ready  for  the  act  of  secretion  and  is 
thrown  into  activity  by  a  nervous  impulse  from  the  central 
nervous  system.  We  speak  of  this  action  of  a  nerve  upon 
the  organ  in  which  its  fibers  end  as  stimulation  and  that 
property  of  an  organ  in  virtue  of  which  it  may  be  aroused 
by  a  stimulus  as  irritability. 

All  the  working  organs  of  the  body  (in  contradistinction 
to  the  supporting  organs,  p.  35)  are  in  this  sense  irritable, 
and  most  of  them  receive  nerves  which  set  them  to  work. 
Irritable  tissues  may,  however,  be  stimulated  by  other  means 
than  by  nervous  impulses.  Of  these  means  an  electric  shock 
is  the  most  familiar;  others  are  the  sudden  application  of 
heat,  the  presence  of  certain  substances  in  the  blood,  and 
even  a  sharp  blow. 


46  THE  HUMAN  MECHANISM 

We  have  now  to  inquire  what  it  is  that  happens  in  the 
gland  when  it  is  stimulated  by  a  nervous  impulse. 

5.  The  response  of  the  gland  to  stimulation  by  its  nerve. 
The  visible  result  of  stimulation  of  the  gland  is  the  discharge 
of  saliva  into  the  mouth.  Something  must  have  happened  in 
the  gland  which  has  led  to  the  passage  of  water  and  other 
substances  from  the  blood  (and  lymph)  through  the  gland 
cells  into  the  duct.  But  something  more  has  happened,  for 
saliva  contains  several  substances  which  are  not  found  in  the 
blood.  The  gland  has  evidently  contributed  something  to  the 
saliva.  How  were  these  contributions  to  the  secretion  made? 


FIG.  32.   Diagram  showing  the  granules  in  a  resting  gland  (A)  and  in  a 
worked  alveolus  of  a  gland  (E) 

When  a  gland  has  been  resting  for  some  time  microscopic 
study  shows  that  the  cytoplasm  of  its  cells  becomes  loaded 
with  small  granules,  at  times  so  numerous  as  to  obscure  the 
nucleus  itself.  As  secretion  goes  on  these  granules  disappear 
from  the  cell,  presumably  contributing  something  to  the 
secretion.  If  the  secretion  continue  for  several  hours,  it  is 
found  that  the  granules  have  disappeared  and  that  the  cell 
is  often  distinctly  smaller  in  size  than  before  secretion  began. 

The  "  resting "  gland  is  therefore  by  no  means  an  idle 
gland,  but  gradually  stores  within  its  cytoplasm  something 
in  the  form  of  granules,  which  under  the  influence  of 
nervous  impulses  or  other  forms  of  stimulation  more  or  less 
rapidly  disappears  in  the  secretion. 

6.  Activity  of  the  gland  involves  chemical  change  within  its 
cells.  It  might  be  supposed  that  the  granules  manufactured 
during  rest  are  merely  dissolved  or  washed  out  of  the  cells 


THE  WORK  OF  ORGANS  AND  CELLS  47 

in  the  copious  stream  of  water  and  salts  which  during  secre- 
tion passes  through  from  the  blood  and  lymph  to  the  duct. 
If  this  were  so,  it  would  be  possible  to  dissolve  from  the 
gland  a  substance  exhibiting  in  general  the  same  properties 
as  the  secretion  itself.  But  this  is  not  generally  the  case. 
Extracts  of  fresh  glands  commonly  fail  to  exhibit  the  char- 
acteristic properties  of  normal  secretions,  although  these  ex- 
tracts may  often  be  changed  by  chemical  means  into  the 
elements  of  the  secretion.  We  are  therefore  compelled  to 
believe  that  the  activity  of  a  gland  means  something  more 
than  the  mere  discharge  of  previously  stored  substances ; 
that  is  to  say,  the  material  of  the  granules  in  the  resting 
cells  is  not  simply  set  free  when  the  gland  secretes,  but  is 
at  the  same  time  chemically  changed.  In  the  digestive  juices, 
for  example,  we  have  active  substances  called  enzymes,  which, 
it  has  been  shown,  are  derived  from  other  substances,  called 
zymogens,  in  the  gland  cells.  The  chemical  change  from  the 
one  into  the  other  is  as  essential  to  the  process  of  secretion 
as  is  the  visible  flow  from  the  duct. 

These  facts  then  present  to  us  the  picture  of  the  cell  as 
the  working  or  physiological  unit,  as  we  have  already  seen 
that  it  is  the  anatomical  unit  of  the  gland  (p.  32).  The 
work  of  the  gland  is  the  sum  of  the  work  of  its  constituent 
cells.  During  the  period  of  rest  these  cells  manufacture 
from  the  blood  zymogens  or  other  substances  which  they 
store  away  in  the  form  of  granules  within  their  cytoplasm. 
When  they  are  stimulated  by  the  nervous  impulse  a  chemical 
change  takes  place  in  them,  the  zymogens  are  changed  to 
enzymes  and  other  substances,  and  these,  together  with  the 
water,  salts,  etc.,  derived  from  the  blood,  form  the  secretion. 

7.  Physiology  of  muscular  contraction.  At  first  sight  mus- 
cles and  glands  seem  to  differ  in  action  or  function  no  less 
than  in  form  and  structure.  No  two  acts  are  apparently 
more  unlike  than  lifting  a  weight  by  the  muscles  of  the 
arm  and  the  secretion  of  saliva  by  the  salivary  glands.  But 


48  THE  HUMAN  MECHANISM 

beneath  obvious  and  important  differences  there  are  profound 
and  fundamental  similarities  in  the  processes  which  occur  in 
the  two  organs  during  activity.  Like  the  gland,  the  muscle 
is  set  to  work  or  stimulated  by  a  nervous  impulse ;  its  con- 
traction is  accompanied  by  an  increased  blood  supply;  and, 
most  important  of  all,  the  work,  or  contraction,  is  accom- 
panied —  indeed,  preceded  —  by  chemical  changes  much  more 
profound  than  that  of  the  transformation  of  zymogen  into 
enzyme.  These  chemical  changes  supply  the  power  for  the 
work. 

That  some  chemical  change  has  taken  place  when  the 
muscle  contracts  is  proved  by  the  fact  that  certain  new  sub- 
stances then  make  then*  appearance  in  the  muscle  and  are 
given  off  to  the  blood  flowing  through  it.  The  most  impor- 
tant of  these  are  carbon  dioxide,  the  gas  which  is  formed 
whenever  wood  or  coal  is  burned,  and  an  acid  substance 
known  as  lactic  acid.  These  substances  were  not  present 
in  the  resting  muscle,  or  else  were  present  in  very  small 
quantities.  With  the  act  of  contraction  relatively  large 
quantities  of  them  make  their  appearance.  They  are  gener- 
ally spoken  of  as  waste  products,  and  it  is  known  that  they 
are  the  result  of  a  chemical  change  in  the  muscle  fiber,  or  cell, 
precisely  as  the  enzymes  are  the  result  of  chemical  changes 
in  gland  cells.  Just  as  glandular  activity  produces  an  out- 
put called  a  secretion,  so  muscular  activity  produces  an 
output  consisting  of  substances  usually  described  as  waste 
products. 

8.  The  storage  of  fuel  within  the  muscle  fiber.  The  source 
of  the  carbon  dioxide  and  lactic  acid  produced  by  the  active 
muscle  must  in  the  long  run  be  the  matter  taken  into  the 
body  in  the  form  of  food.  After  undergoing  in  the  stomach 
and  intestine  relatively  simple  changes,  which  do  not  pro- 
foundly affect  its  chemical  constitution,  this  food  is  absorbed 
into  the  blood  and  through  this  channel  delivered  to  the 
cells.  Thus  far,  however,  the  food  material  does  not  differ 


THE  WORK  OF  ORGANS  AND  CELLS  49 

greatly  from  the  food  as  swallowed.  Especially  to  be  noted 
is  the  fact  that  it  does  not  undergo  sudden  and  profound 
chemical  change.  When,  on  the  other  hand,  a  muscle  is 
stimulated  to  contraction,  there  occurs  in  it  a  chemical 
change  requiring  less  than  the  hundredth  of  a  second  for  its 
completion.  This  of  course  suggests  the  chemical  change  in 
gunpowder  or  dynamite.  Obviously  the  food  delivered  by 
the  blood  to  the  muscle  fiber  has  been  transformed  in  the 
fiber  into  something  more  unstable,  something  capable  of 
a  very  sudden  chemical  change.  The  meat,  bread,  butter, 
potatoes,  and  the  like  have  been  changed  into  something 
comparable  to  the  phosphorus  in  a  match  or  the  gunpowder 
in  a  percussion  shell.  This  unstable  material  has  not  been 
demonstrated  as  granules  or  other  visible  material  within  the 
cell,  as  have  the  zymogen  granules  of  a  gland;  nor  has  it 
been  extracted  from  the  cell,  as  have  mucin  and  enzymes ; 
but  the  facts  force  the  conclusion  that,  like  the  gland  cell, 
the  muscle  fiber  has  used  its  period  of  rest  to  make  and  store 
within  itself  an  unstable  compound  which  undergoes  upon 
the  application  of  a  stimulus  a  very  sudden  chemical  change. 
This  unstable  compound  we  may  call  the  fuel  substance  or 
the  fuel  of  the  fiber. 

9.  Available  and  reserve  fuel.  The  analogy  of  a  match  is 
useful  to  make  clear  these  fundamental  conceptions  of  mus- 
cular activity.  The  phosphorus  on  the  head  of  the  match  is 
the  unstable  fuel  substance ;  the  friction  of  the  match  when 
it  is  rubbed  against  a  rough  surface  is  the  stimulus,  which 
is  followed  by  a  sudden  chemical  change  in  the  fuel  when 
the  match  "  goes  off."  At  this  point,  however,  the  analogy 
ends;  for  when  a  second  stimulus  is  applied  to  a  muscle 
'within  one  tenth  of  a  second,  there  is  a  second  contraction, 
and  in  this  second  contraction  there  is  the  same  sudden 
chemical  change  in  the  fuel ;  moreover,  this  stimulation  may 
be  repeated  over  and  over  again  with  like  results.  Even 
more  striking  is  the  fact  that  the  same  thing  is  true  of  a 


50  THE  HUMAN  MECHANISM 

muscle  removed  from  the  body  and  consequently  shut  off 
from  access  to  new  fuel  supply  in  the  blood  flowing  through 
it.  Such  an  excised  muscle  will  give  a  long  series  of  con- 
tractions upon  the  repeated  application  of  stimuli.  With  the 
match  or  the  percussion  cap,  on  the  other  hand,  such  repeated 
discharges  would  not  occur,  for  the  entire  stock  of  fuel  is 
used  up  with  each  discharge. 

In  order  to  explain  these  facts,  it  is  commonly  assumed 
that  the  fuel  substance  of  the  muscle  fiber  exists  in  two 
forms:  the  one  unstable  and  ready  to  be  discharged  by  the 
stimulus ;  the  other  and  larger  part  incapable  of  being  dis- 
charged by  the  stimulus,  but  rapidly  providing,  after  each 
discharge,  the  material  to  make  good  the  loss  of  unstable  fuel. 
We  may  speak  of  the  one  as  the  available  or  unstable  fuel  and 
of  the  other  as  the  reserve  fuel. 

10.  The  chemical  change  of  unstable  fuel  into  waste  prod- 
ucts involves  cleavage  and  oxidation.  Although  our  present 
knowledge  is  inadequate  to  the  full  understanding  of  the 
chemical  changes  in  the  muscle  during  activity,  it  can  at 
least  be  stated  that  changes  of  two  kinds  are  involved, 
namely  oxidation  arid  cleavage. 

Oxidation  is  the  union  of  the  material  with  oxygen,  one 
of  the  gases  of  the  atmosphere.  When  carbon  (charcoal)  is 
burned,  for  example,  it  disappears  by  uniting  with  oxygen 
to  form  the  colorless  gas,  carbon  dioxide ;  when  hydrogen  is 
burned,  it  unites  with  oxygen  to  form  water;  or  if  a  chem- 
ical compound  of  carbon  and  hydrogen  (for  example,  kero- 
sene) is  burned,  its  carbon  unites  with  oxygen  to  form  carbon 
dioxide,  while  its  hydrogen  unites  with  oxygen  to  form  water. 
Conversely,  when  we  find  that  the  products  of  any  chemical 
change  contain  more  oxygen  than  the  original  substance,  we 
infer  that  the  change  is  a  combustion  or,  as  it  is  generally 
called,  an  oxidation. 

The  second  kind  of  chemical  change,  cleavage,  takes  place 
without  the  addition  of  oxygen  or,  indeed,  of  any  other 


THE  WORK  OF  ORGANS  AND  CELLS  51 

chemical  element,  .except  that  water  is  often  added  to  the 
material  changed.  In  this  process  the  combination  of  differ- 
ent atoms  which  makes  the  compound  is  broken  and  the 
molecule  is  split  into  two  or  more  molecules.1  Thus  new 
compounds  or  substances  are  formed. 

In  the  muscle  fiber  both  these  changes  occur  during  con- 
traction. Many,  perhaps  the  majority  of  physiologists,  now 
think  that  the  stimulus  to  the  muscle  fiber  (nerve  impulse, 
electric  shock)  first  causes  a  cleavage  of  the  unstable  fuel 
of  the  fiber  into  lactic  acid  and  possibly  other  products  and 
that  this  cleavage  is  the  cause  of  the  contraction ;  under 
normal  conditions  this  is  followed  by  an  oxidation  of  the 
lactic  acid  to  carbon  dioxide  and  water 

C3H6O3  +  3  O2  =  3  CO2  +  3  H2O. 

On  this  view  the  cleavage  takes  place  very  suddenly  (per- 
haps requiring  less  than  the  hundredth  of  a  second),  while 
the  oxidation  which  follows  requires  several  seconds  or  even 
minutes  for  completion ;  indeed,  before  it  is  complete,  some 
of  the  lactic  acid  may  have  passed  out  of  the  muscle  fiber 
into  the  lymph  and  blood.  Some  of  the  facts  supporting  this 
view  are  the  following:  the  lactic  acid  produced  within  the 
muscle  during  contraction  increases  with  the  intensity  of  the 
work ;  the  amount  of  it  found  after  contraction  is  greater 
when  the  supply  of  oxygen  from  the  blood  is  diminished  or 
cut  off ;  and,  finally,  lactic  acid  disappears  from  the  muscle 
more  rapidly  after  contraction,  when  the  blood  is  well  sup- 
plied with  oxygen,  than  when  it  is  deficient  in  that  element. 

1  Matter  is  composed  of  atoms  of  chemical  elements  ;  these  atoms  are 
combined  or  bound  together  to  form  molecules.  A  lump  of  sugar,  for  ex- 
ample, would  be  composed  of  an  inconceivable  number  of  molecules  of  sugar, 
and  each  molecule  would  consist  of  six  atoms  of  carbon,  twelve  atoms  of 
hydrogen,  and  six  atoms  of  oxygen  bound  together  in  chemical  combination. 
Sugar  may  undergo  cleavage  into  lactic  acid  by  splitting  its  molecule  of 
twenty-four  atoms  into  two  molecules  of  twelve  atoms.  The  chemist  ex- 
presses this  by  the  following  equation : 


52  THE  HUMAN  MECHANISM 

Let  us  not  lose  sight  of  the  central  fact.  The  activity  of  a 
muscle  fiber,  like  the  activity  of  a  gland  cell,  is  the  result  of  a 
chemical  change  within  the  cell.  In  both  cases  the  food  mate- 
rial derived  from  the  blood  is  transformed  into  something  else 
and  activity  is  accompanied  by  the  production  of  new  sub- 
stances. In  the  case  of  the  gland  these  new  substances,  or 
part  of  them,  go  to  form  essential  constituents  of  the  secre- 
tion, and  we  see  at  once  the  end  secured  by  the  chemical 
change.  In  the  case  of  the  muscle  the  end  is,  at  first  sight, 
not  so  clear.  The  substances  formed  are  not  of  obvious  use 
to  the  body,  and  we  have  now  to  inquire  how  this  chemical 
change  serves  the  purpose  of  producing  a  muscular  contraction. 

11.  Relation  of  the  chemical  changes  to  the  work  of  muscular 
contraction.  It  is  a  familiar  fact  that  chemical  changes  often 
yield  power  for  work.  The  explosion  of  dynamite  (a  cleavage 
change),  for  example,  will  shatter  large  masses  of  rock ; 
the  oxidation  of  coal  in  a  locomotive  engine  supplies  the 
power  to  move  a  heavy  train  of  cars.  In  both  cases  waste 
products  are  produced,  and  in  the  change  which  produces 
them  power  is  liberated;  but  in  order  that  this  power  may 
be  utilized  to  do  work,  some  mechanism  is  needed  to  apply 
it  to  the  desired  end.  The  burning  of  coal  in  an  open  grate 
liberates  power,  but  in  the  absence  of  any  mechanism  adapted 
to  that  purpose,  it  does  no  work ;  the  same  coal  burned  under 
the  boiler  of  an  engine,  with  its  mechanism  of  boiler,  piston, 
driving  rod,  and  wheels,  moves  the  train  of  cars. 

So  it  is  with  the  muscle.  Within  the  cytoplasm  of  the 
fiber  are  the  myofibrils  (p.  35),  and  there  are  convincing 
reasons  for  believing  that  the  combination  of  myofibril  and 
sarcoplasm  constitutes  the  mechanism  of  the  muscle  fiber. 
The  power  liberated  by  the  cleavage  change  acts  upon  this 
mechanism,  causing  the  shortening  and  thickening  of  the 
myofibrils,  whereby  a  pull  is  exerted  on  the  tendon.  Whether 
the  subsequent  oxidative  changes  also  contribute  power  for 
the  work  or  merely  produce  heat  is  still  an  open  question. 


THE  WORK  OF  ORGANS  AND  CELLS  53 

12.  Heat  production  by  the  working  muscle.    One  other 
point  of  similarity  between  the  working  muscle  fiber  and  the 
working  steam  engine  should  be  pointed  out;  namely,  that 
both  produce  heat.  It  is  a  familiar  fact  that  muscular  activity 
makes  us  feel  warm.    This  is  the  direct  result  of  the  libera- 
tion of  heat  by  the  oxidations  within  the  working  muscle 
fiber.    The  same  thing  is  true  of  the  steam  engine,  the  liber- 
ated heat  going  in  that  case  to  warm  the  engine  or  passing 
away  in  the  gases  which  escape  from  the  smokestack,  steam 
vents,  etc.     It  is  important  that  the  student  of  physiology 
bear  clearly  in  mind  this  feature  of  muscular  action,  since 
the  active  muscles  not  only  supply  power  for  work  but  also 
the  heat  necessary  to  maintain  the  temperature  of  the  body, 
and  no  muscle  can  be  thrown  into  contraction  without  liber- 
ating a  certain  amount  of  heat.    For  a  full  discussion  of  this 
matter  see  Chapter  XII. 

13.  The  repair  and  maintenance  of  the  cellular  nu 

Thus  far  we  have  considered  only  those  chemical  activities 
of  gland  and  muscle  cells  which  are  directly  concerned  with 
secretion  and  contraction  or  which  prepare  the  cell  for  the 
performance  of  these  functions.  This  is  only  a  part,  how- 
ever, of  the  work  of  living  cells,  for,  like  all  machines,  cells 
may  be  injured  by  overwork  or  by  accident,  and  their  parts 
(nucleus,  cytoplasm,  fibrils,  etc.)  must  be  kept  in  working 
order.  Just  here  the  living  mechanism  differs  from  the  life- 
less engine,  for  the  living  mechanism  is  itself  capable  of 
repairing  damage  to  itself.  The  locomotive  must  be  sent  to 
the  shops  and  be  repaired  by  work  done  upon  it  by  other 
machines;  if  the  boiler  rusts,  it  must  be  taken  out  and  a 
new  one  put  in ;  if  the  wheels  wear  unevenly,  they  must  be 
made  true  again  by  turning  in  a  lathe  or  new  ones  must 
be  substituted ;  when  the  grate  burns  out,  a  new  one  must  be 
put  in  its  place.  The  living  cell,  on  the  other  hand,  itself 
makes  these  repairs  from  certain  constituents  of  the  same 
food  out  of  which  fuel  and  zymogen  granules  are  made,  and 


54  THE  HUMAN  MECHANISM 

it  does  this  by  other  chemical  activities  than  those  we  have 
described  and  about  which  we  possess  only  fragmentary 
knowledge  at  present.  In  picturing  to  ourselves  the  activi- 
ties of  these  living  mechanisms  we  must  include  all  these 
chemical  processes,  those  of  maintenance  and  repair  as  well 
as  those  concerned  immediately  with  the  performance  by 
each  cell  of  its  own  special  functions,  such  as  secretion  by 
a  gland  and  contraction  by  a  muscle. 

14.  Recapitulation.  We  have  traced  the  character  of  the 
work  done  in  the  case  of  the  gland  and  the  muscle  and  have 
found  that  it  is  fundamentally  the  work  of  the  cells  of  which 
the  organs  are  composed.  The  cells  of  other  organs  are  simi- 
larly constructed  to  do  other  kinds  of  work,  and  the  character 
of  their  chemical  changes  and  of  the  mechanisms  for  utilizing 
power  varies  accordingly ;  all,  however,  showing  the  same 
fundamental  plan  of  working  engines.  The  body  is  a  com- 
munity of  groups  of  cells  of  different  kinds,  each  kind  doing 
some  work  more  or  less  peculiar  to  itself.  In  addition  to  the 
two  groups  (gland  and  muscle  cells)  which  we  have  studied, 
there  are  nerve  cells  in  the  brain,  spinal  cord,  and  elsewhere ; 
cells  which  make  blood  corpuscles;  cells  which  keep  in  repair 
the  connective  tissues  (bone,  gristle,  tendon,  and  ligament)  ; 
and  many  more,  such  as  cells  which  manufacture  or  them- 
selves form  the  lining  of  free  surfaces,  like  the  skin,  the  ali- 
mentary tract,  the  air  passages,  etc.  The  sum  total  or  net 
result  of  the  activities  of  these  and  other  cells  makes  up  the 
work  of  the  body  as  a  whole.  The  work  of  the  body  —  the 
human  organism,  the  human  mechanism — is  thus  the  outcome 
or  resultant  of  the  work  of  its  different  component  cells. 


CHAPTER  V 
WORK  AND  FATIGUE 

While  it  is  true,  as  shown  in  the  last  chapter,  that  ca- 
pacity for  work  is  one  of  the  principal  characteristics  of  the 
human  body,  no  experience  of  daily  life  is  more  familiar 
than  that  work  is  followed  by  fatigue.  This  is  true  both 
of  individual  organs  and  of  the  organism  as  a  whole ;  for 
fatigue  may  be  either  local,  as  when  some  one  muscle  is 
tired  from  hard  work,  or  general,  as  when  weariness  affects 
all  organs  —  those  which  have  been  resting  as  well  as  those 
which  have  been  working. 

We  use  the  word  "fatigue"  in  two  different  senses,  and  it 
is  important  that  a  distinction  be  clearly  made  between  them. 
In  the  one  sense  the  word  means  the  diminution  of  working 
capacity  due  to  work.  In  testing  one's  strength  of  grip  or  of 
back  a  second  test,  if  made  immediately,  shows  less  work  done 
than  at  the  first  test,  and  this  is  true  whether  or  not  we 
are  conscious  of  fatigue  or  of  diminished  working  power.  If, 
however,  a  certain  time  be  allowed  for  rest,  the  second  test 
will  give  as  good  results  as  the  first. 

In  the  other  sense  the  word  refers  to  the  feeling  of  fatigue 
which  frequently,  though  not  always,  accompanies  the  dimi- 
nution of  working  power.  We  may  "  feel  tired  "  when  we 
have  been  doing  nothing,  and  conversely,  under  the  influence 
of  excitement  or  other  causes  we  may  experience  no  feeling 
of  fatigue  even  when  we  are  near  the  limit  of  our  working 
power.  Often  in  an  exciting  game  the  players  do  not  know 
at  the  time  that  they  are  tired  or  even  that  their  working 
power  is  lessened;  and  stories  are  told  of  soldiers  in  hasty 

56 


56 


THE  HUMAN  MECHANISM 


retreat  who  feel  that  they  must  "  drop  in  their  tracks " 
until  the  discharge  of  musketry  close  behind  stimulates 
them  to  move  faster  than  ever. 

The  feeling  of  fatigue  has  its  seat  in  the  nervous  system, 
and  its  study  must  be  postponed  until  we  have  learned  some- 
thing of  the  physiology  of  the  brain  and  spinal  cord.  In  the 
present  chapter  we  are  not  immediately  concerned  with  this 
side  of  the  question,  but  rather  with  the  diminution  of  work- 
ing power  produced  by 
work.  Such  fatigue 
must  be  measured  not 
by  our  sensations  but 
by  the  work  accom- 
plished, whether  that 
work  be  physical  or 
mental.  And  as  we 
studied  the  physiology 
of  work  in  its  simplest 
form  in  a  single  work- 
ing organ,  such  as  a 
muscle  or  a  gland,  so  we 

can  best  begin  our  study 
FIG.  33.   Diagram  of  apparatus  for  recording  •    •  ?  j  • 

successive  muscular  contractions  01   diminished  working 

power  or  fatigue  in  one 
of  these  same  organs,  namely,  the  skeletal  muscle. 

1.  Fatigue  of  an  isolated  muscle  and  of  a  muscle  with  in- 
tact circulation.  The  course  of  fatigue  in  a  muscle  is  best 
studied  by  causing  the  muscle  to  contract  to  its  utmost,  at 
regular  intervals  of  time,  against  the  resistance  of  a  suitable 
spring.  If  now  we  record  the  height  of  each  contraction,  we 
obtain  a  series  which  shows  at  once  the  effect  of  the  work 
on  the  working  power ;  that  is,  the  course  of  fatigue.  Fig.  33 
gives  a  diagram  of  the  arrangement  of  such  an  experiment 
with  an  isolated  muscle ;  that  is,  a  living  muscle  detached 
from  the  rest  of  the  body.  One  tendon  is  made  fast  in  a 


WORK  AND  FATIGUE 


57 


rigid  clamp,  while  the  other  is  attached  to  the  spring,  which 

is  stretched  by  the  contraction  when  the  muscle  is  stimulated. 

The  length  of  the  line  written   by  the   lever  AB  records 

what  the  muscle  is  capable 

of  doing  at  the  time ;    and 

if  the  records  of  successive 

contractions    are    made    on 

the    smoked    surface    of    a 

slowly  revolving   drum,    as 

in   the  figure,   we   have    at 

once  a  record  of  the  course 

of  fatigue. 

Such  fatigue  tracings  may 
also  be  taken  from  a  muscle 
within  the  body,  and  hence 
with  its  circulation  intact. 
Thus  the  work  of  the  biceps 
muscle  in  bending  the  arm 
at  the  elbow  (Fig.  23)  may 
be  recorded  by  instruments 
essentially  similar  to  that 
used  with  the  excised  muscle. 
In  Fig.  34  we  have  repro- 
duced a  tracing  of  this  kind. 

It  is  quite  evident  that  a 
continuous  line  joining  the 
highest  points  reached  by 
the  several  contractions  will 
represent  graphically  the 
course  of  fatigue,  and  in 
Fig.  35  the  line  a  represents 
this  so-called  "  curve  of 
fatigue "  in  the  experiment 
whose  results  are  given  in 
Fig.  34.  It  falls  off  at  first 


FIG.  34.   Record  of  the  successive  con- 
tractions of  the  flexor  muscles  of  the 
elbow  joint 

Showing  the  gradual  decrease  in  working 
power  to  a  fatigue  level.  The  muscle  con- 
tracted once  every  three  seconds  against 
the  resistance  of  a  strong  spring,  which 
was  stretched  each  time  as  far  as  the 
strength  of  the  muscle  permitted 


58 


THE  HUMAN  MECHANISM 


rather  rapidly,  then  more  and  more  slowly,  until  at  last  it 
becomes  parallel  with  the  base  line.  In  other  words,  the 
muscle  in  this  case  finally  finds  a  constant  level  of  working 
power.  This  may  be  called  the  fatigue  level. 

The  broken  line  b  in  Fig.  35  gives  the  result  of  a  fatigue 
tracing  with  the  isolated  muscle.  It  will  be  seen  that  the 
fall  in  the  height  of  contraction  continues  until  at  last  the 
muscle  no  longer  responds  to  stimulation.  The  contrast 
thus  brought  out  between  the  effect  of  work  upon  muscles 


FIG.  35.    Curves  of  fatigue 
a,  from  a  muscle  with  intact  circulation ;  6,  from  an  isolated  muscle 

with  and  those  without  the  circulation  suggests  that  the  cir- 
culation  of  the  blood  through  the  working  organ  in  some 
way  maintains  the  working  power. 

The  height  of  the  fatigue  level  in  the  same  muscle  at 
different  times  is  very  closely  dependent  on  the  rate  at 
which  the  muscle  works.  Thus  with  a  contraction  every 
four  seconds  instead  of  every  three  seconds  the  fatigue 
level  would  be  higher  than  in  Fig.  34;  with  a  contraction 
every  second  it  would  be  much  lower.  When  the  contrac- 
tions come  every  nine  or  ten  seconds  there  is  usually  no 
falling  off  in  the  work  done,  the  time  between  contractions 
being  sufficient  for  the  complete  recovery  of  working  power. 

This  picture  of  fatigue  hardly  agrees  with  our  feeling  of 
fatigue,  for  the  decline  of  working  power  begins  at  once,  or 


WOKK  AND  FATIGUE  59 

at  most  after  a  very  small  number  of  contractions,  whereas 
we  usually  notice  fatigue  only  after  work  has  gone  on  for 
a  considerably  longer  time.  One  does  not  feel  tired  from 
walking,  for  example,  during  the  first  ten  or  twenty  minutes 
of  the  walk.  We  need  not  discuss  here  just  what  makes  us 
unconscious  of  the  beginnings  of  fatigue ;  but  it  is  important 
to  understand  that  whether  we  are  or  are  not  aware  of  its 
presence,  fatigue  is  the  invariable  and  immediate  result  of 
all  muscular  work. 

Weariness  is  simply  the  conscious  feeling  of  fatigue,  but 
fatigue  is  a  physical  condition  of  living  cells  and  organs. 
Moreover,  its  phenomena  are  by  no  means  confined  to 
muscular  work.  When  a  gland  is  stimulated  to  vigorous 
secretion  a  diminution  is  sooner  or  later  noted  in  the  amount 
of  the  secretion,  and  there  is  some  reason  to  believe  that 
nerve  cells  may  also  become  tired  from  continued  activity. 
Fatigue,  then,  in  one  word,  is  a  natural  condition  of  an 
organ  accompanying  work,  and  we  may  proceed  to  inquire 
into  its  exact  cause. 

2.  Waste  products  as  a  cause  of  fatigue.  When  blood 
which  has  been  circulating  through  a  fatigued  muscle  is 
sent  through  a  resting  muscle,  the  resting  muscle  shows 
signs  of  fatigue,  even  though  it  has  itself  done  no  work. 
Apparently  the  blood  has  extracted  from  the  working 
muscle  something  which  has  the  power  of  lessening  the 
working  capacity  of  a  fresh  muscle. 

The  same  thing  is  illustrated  by  another  experiment.  A 
muscle  which  is  deprived  of  its  circulation  (for  example,  by 
clamping  its  arteries  and  veins)  is  fatigued  by  vigorous  work ; 
it  is  then  found  that  although  when  left  to  itself  a  slight  re- 
covery takes  place,  this  recovery  is  much  more  marked  if  we 
first  pass  through  its  blood  vessels  a  weak  solution  of  salt 
Here  no  food  is  supplied ;  the  salt  solution  has  only  removed 
something  from  the  fatigued  muscle,  which,  in  consequence  of 
this  treatment,  recovers  some  of  its  working  power. 


60  THE  HUMAN  MECHANISM 

Again,  the  mere  exposure  of  a  resting  muscle  to  blood 
containing  lactic  acid  or  to  blood  heavily  charged  with 
carbon  dioxide  (CO2)  produces  the  condition  of  fatigue. 
Now  in  the  last  chapter  it  has  been  shown  that  both  lactic 
acid  and  carbon  dioxide  are  waste  products  of  muscular 
activity;  and  these  and  other  facts  have  led  to  the  view, 
now  generally  received,  that  the  waste  products  of  the 
active  organ  interfere  with  the  work  of  the  organ  and  so 
constitute  one  of  the  main  causes  of  fatigue.  It  is  ap- 
parently for  this  reason  that  the  injection  of  an  extract  of 
worked  muscle  fatigues  fresh  muscle,  for  the  extract  con- 
tains waste  products.  It  is  for  the  same  reason  that  wash- 
ing out  a  fatigued  muscle  with  salt  solution  produces  partial 
recovery,  for  the  waste  products  of  activity  are  in  this  way 
partially  removed.  We  can  also  understand  why  fatigue 
always  accompanies  vigorous  work.  Waste  products  then 
necessarily  accumulate  and  clog  the  living  mechanism  be- 
cause they  cannot  be  removed  by  the  blood  as  fast  as  they 
are  formed  by  the  muscle  cells.  No  fatigue  occurs  with 
only  a  single  contraction  every  ten  seconds  or  more  be- 
cause between  contractions  sufficient  time  is  given  to  insure 
the  complete  removal  of  wastes. 

3.  Loss  of  fuel  in  the  working  muscle  as  a  cause  of  fatigue. 
The  blood,  however,  not  only  removes  the  wastes  but  also 
brings  new  food  and  oxygen  with  which  the  muscle  makes 
good  the  loss  of  fuel;  and  it  may  well  be  —  although  it  is 
not  absolutely  proved  —  that  recovery  from  fatigue  depends 
upon  both  of  these  good  offices  of  the  blood.  We  have 
certainly  one  well-established  cause  of  fatigue,  namely,  the 
presence  of  the  waste  products  of  activity ;  and  we  recog- 
nize the  probability  that  the  depletion  of  fuel  may  also 
contribute  to  the  result.  But  whether  the  first  of  these 
causes  alone  is  sufficient  to  explain  it,  or  whether  both 
work  together,  we  can  understand  that  the  maintenance 
of  a  good  blood  supply  is  of  the  first  necessity  and  that 


WORK  AND  FATIGUE  61 

undue  fatigue  can  be  avoided  only  by  working  at  a  moder- 
ate rate.  It  is  an  old  and  physiologically  true  saying  that 
"  it  is  the  pace  that  kills." 

4.  Explanation  of  the  fatigue  level.    In   the   experiment 
with  the  isolated  muscle  no  waste  products  were  removed 
nor  were  new  food  and  oxygen  supplied;  hence  the  wastes 
in  the  muscle  increased  with  each  contraction,  until  at  last 
their  accumulation  prevented  all  contraction.    In  the  normal 
muscle  the  wastes  likewise  accumulate  for  a  time ;  and  this 
is  why  the  curve  of  work  at  first  falls  (Fig.  34).     It  does 
not  continue  to  fall,  because  as  the  wastes  within  the  muscle 
increase    in    amount,   the    blood   carries   more   and  more   of 
them  away  in  a  given  time.    The  quantity  of  waste  removed 
thus  continues  to  increase  until  the  same  quantity  is  carried 
away   from   the    muscle   between    two    contractions    as    the 
muscle  produces  with  each  contraction.    When  this  happens 
no  further  accumulation  of  waste  is  possible  and  the  fatigue 
level  is  established. 

5.  General  fatigue  resulting  from  muscular  activity.    Every- 
one knows  that  after  a  day's  tramp   it  is  not  simply  the 
worked  muscles  which  are   unfit  for  good   work,  but  that 
the  brain,  too,  is  tired,  for  hard  mental  work  is  then  dim- 
cult  or  well-nigh  impossible ;    and  it  is   generally  the  fact 
that  long-continued  muscular  work  fatigues  the  brain  more 
than  brain  (mental)  work  itself.     The  obvious  explanation 
of  this  fact  is  that  the  waste  products  of  muscular  activity 
have  accumulated  in  the  blood  more  rapidly  than  the  body 
can  get  rid  of  them,  and  so  have  fatigued  the  other  tissues, 
including  the  nerve  cells  of  the  brain,  just  as  the  injection 
of  the  extract  of  a  tired  muscle  lessens  the  working  power 
of  a  fresh  muscle.    No  doubt  these  same  waste  products  may 
similarly  fatigue  gland  cells;  for  experience  seems  to  show 
that  the  secretion  of  digestive  juices  is  not  so  active  when 
one  is  suffering  from  muscular  fatigue  and  that  it  is  not 
wise  to  eat  heavy  meals  when  one  is  tired  out.    We  can  also 


62  THE  HUMAN  MECHANISM 

understand  why  long-continued,  vigorous  muscular  action 
produces  marked  fatigue  in  nerve  cells  and  gland  cells, 
while  the  activity  of  the  latter  produces  only  inappreciable 
fatigue  in  the  muscles;  for  the  amount  of  chemical  change 
and  the  production  of  wastes  are  far  greater  in  the  case  of  mus- 
cular work  than  in  that  of  nervous  or  glandular  activity. 

6.  The  analogy  of  the  engine.  In  previous  chapters  we 
have  compared  the  living  body  with  a  machine  or  locomotive 
engine ;  both  do  work,  and  both  obtain  the  power  for  work 
from  the  chemical  changes  in  food  or  fuel.  What  we  have 
now  learned  about  fatigue  suggests  an  extension  of  the 
same  comparison.  Every  locomotive  suffers  impairment  of 
its  working  power  with  use,  and  special  measures  are  taken 
to  limit  this  impairment  as  much  as  possible;  the  gases  and 
smoke  are  carried  away  at  once  by  the  chimney  or  smoke- 
stack ;  the  furnace  is  provided  with  a  grate  so  that  the  ashes 
shall  not  accumulate  and  shut  off  the  draft ;  the  bearings  are 
oiled  and  foreign  matters  removed;  finally,  as  the  consump- 
tion of  fuel  goes  on,  the  loss  is  made  good  by  stoking. 

The  continuance  of  the  work  of  the  engine  requires  two 
things  —  fresh  supplies  of  fuel  and  the  removal  of  wastes. 
Obviously  the  blood  performs  these  same  offices  for  the  cell. 
It  supplies  to  the  cell  fuel  (food)  from  the  alimentary  canal 
and  oxygen  from  the  lungs  and  it  carries  away  the  waste. 
Provision  is  thus  made  to  maintain  the  human  machine  in 
working  order  and  good  condition  during  its  activity.  If  the 
blood  flows  too  slowly  through  the  muscle,  the  same  thing 
happens  as  in  the  locomotive  when  the  fireman  neglects  to 
rake  the  fire  or  to  put  on  new  fuel ;  the  efficiency  both  of  the 
human  engine  and  of  the  locomotive  may  be  impaired  either 
by  the  undue  accumulation  of  the  waste  products  of  its  own 
activity  or  by  the  neglect  to  supply  proper  food  or  fuel. 


CHAPTER  VI 

THE  INTEKDEPEKDENCE  OF  OKGANS  AND  OF  CELLS 
INTERNAL  SECRETIONS 

1.  The  products  of  cellular  activity  not  necessarily  harm- 
ful. We  have  now  learned  that  the  active  living  cells  of 
the  body  are  the  seat  of  chemical  changes  which  produce 
new  substances ;  that  the  accumulation  of  these  products  of 
activity  often  limits  the  working  power  of  the  cells  in  which 
they  are  produced,  and  may  even  depress  the  activity  of 
other  cells  to  which  they  are  carried  by  the  blood.  In  the 
case  of  the  skeletal  muscles  we  have  spoken  of  the  carbon 
dioxide,  the  sarcolactic  acid,  etc.  as  "waste  products,"  mean- 
ing thereby  that  they  are  incapable  of  serving  as  sources  of 
power  for  the  work  of  the  muscle;  and  this  term,  together 
with  the  fact  that  they  constitute  one  cause  of  fatigue,  is  apt 
to  mislead  us  into  supposing  that  they  can  be  of  no  further 
use  to  the  body  or,  even  more,  that  they  are  necessarily 
harmful  and  that  their  presence  in  the  blood  is  objectionable. 

These  conclusions,  however,  do  not  necessarily  follow  from 
the  facts.  It  does  not  even  follow  that  a  substance  which 
produces  fatigue  for  that  reason  serves  no  useful  purpose. 
Most  adults  can  recall  times  when  because  of  long-continued 
application  to  mental  work  or  because  of  worry  or  other 
nervous  strain  they  have  become  overexcitable  and  restless 
and  have  been  unable  to  obtain  the  sleep  of  which  the  body 
as  a  whole  stands  in  need.  At  such  times  sleep  is  often 
best  secured  by  producing  general  fatigue  through  muscular 
work.  The  waste  products,  by  their  very  act  of  fatiguing 
the  overexcited  nerve  cells,  may  be  of  service  to  the  body 

68 


64 


THE  HUMAN  MECHANISM 


as  a  whole ;  and  it  is  probably  true  that  not  only  in  such 
abnormal  conditions  but  also  in  the  daily  conduct  of  life  the 
fatigue  of  moderate  muscular  activity  contributes  its  share 
toward  inducing  healthful  and  refreshing  slumber. 

Thus  far  we  have  considered  the  chemical  activities  of 
each  organ  as  contributing  to  the  work  of  the  organ  in 
which  they  occur  and,  because  of  the  accumulation  of  waste 

products,  as  the  occasional 
cause  of  undue  interference 
with  efficient  activity,  both  in 
the  working  organ  and  else- 
where. And  yet  the  familiar 
case  which  we  have  just  cited 
suggests  another  view  of  the 
matter.  The  products  of  the 
chemical  activity  of  one  organ 
may  be  of  service  to  other 
organs,  and  so  to  the  body 
as  a  whole;  and  while  their 
too  rapid  accumulation  in  the 
FIG.  36.  Cross  section  of  the  thyroid  blood  maJ  be  undesirable, 


gland 


their    presence    in    moderate 


The  cells  secrete  into  the  closed  sacs,  amounts  may  be  beneficial 
^tUS7£±£i££  and  may  contribute  to  the 
the  cells  into  the  lymph  spaces  of  the  normal  environment  of  the 

connective  tissue  ^^        f    .-,       •,      •* 

cells  of  the  body. 

2.  The  thyroid  gland.  This  view  of  the  case  is  strikingly 
emphasized  in  the  physiology  of  the  thyroid  gland  —  a  small 
organ  in  the  neck,  the  two  chief  lobes  of  which  lie  alongside 
the  trachea.  For  a  long  time  its  use  was  not  understood, 
and  at  times  it  was  even  supposed  that  it  plays  no  important 
part  in  the  life  of  the  body  as  a  whole.  It  has  been  found 
by  experiment,  however,  that  removal  of  the  thyroid  is  fol- 
lowed by  a  disease  in  all  respects  similar  to  one  which  had 
long  been  observed  in  human  beings,  especially  in  children; 


INTERDEPENDENCE  OF  ORGANS  65 

and  this  fact  suggested  that  the  disease  is  due  to  the  failure 
of  the  thyroid  to  perform  its  normal  functions. 

The  subject  was  further  cleared  up  by  the  discovery  that 
after  the  removal  of  the  thyroid  in  a  lower  animal  the  disease 
in  question  could  be  prevented  by  feeding  the  animal  thyroids 
or  even  by  giving  to  it  a  certain  substance  extracted  from 
them.  Evidently  the  thyroid  manufactures  and  discharges 
into  the  blood  a  peculiar  substance  necessary  to  the  healthy 
life  of  the  cells  of  the  body ;  and  when  the  gland  fails  to 
manufacture  this  substance  it  can  still  be  supplied  artifi- 
cially by  introducing  it  into  the  blood  by  absorption  from 
the  alimentary  canal. 

3.  Internal  secretions.    In  our  study  of  secretion  in  Chap- 
ter IV  (p.  43)  we  dealt  only  with  glands  which  discharge 
their  principal  products  through  a  duct  into  some  part  of  the 
alimentary  canal;  such  glands  are  the  salivary  glands,  the 
pancreas,  and  the  liver.   Other  glands  send  ducts  to  the  surface 
of  the  body  —  for  example,  the  sweat  glands,  which  discharge 
perspiration  upon  the  skin ;  and  the  lachrymal  glands,  which 
discharge  the  tears  on  the  eyeball.    In  the  case  of  the  thyroid, 
on  the  other  hand,  we  have  an  example  of  an  organ  which, 
like  those  just  mentioned,  manufactures  a  special  substance 
from  the  blood,  but,  having  no  duct,  contributes  the  products 
of  its  manufacture  to  the  blood,  for  the  use  of  other  cells. 
This  process  is  spoken  of  as  internal  secretion,  to  distinguish 
it  from  ordinary  secretion,  in  which  case  something  is  dis- 
charged on  a  free  surface  like  the  skin  or  into  the  alimentary 
canal,  the  nasal  cavity,  or  the  air  passages. 

4.  The  adrenal  glands.    Lying  immediately  above  the  kid- 
neys are  two  small  glandular  organs,  the  adrenals,  which,  like 
the  thyroid,  were  formerly  considered  of  minor  importance. 
It  has  been  shown,  however,  that  these  also  contribute  to 
the   blood   a  most   important   internal    secretion   known  as 
adrenaline.    This  substance  is  manufactured  by  the  gland  cells 
and,  during  their  periods  of  inactivity,  is  stored  within  the 


66 


THE  HUMAN  MECHANISM 


cells,  from  which  it  is  discharged  by  nervous  impulses.  Like 
the  thyroids,  the  adrenals  have  no  ducts ;  but  the  cells  of 
the  gland  come  into  very  close  relation  with  the  unusually 
rich  network  of  blood  capillaries  into  which  the  adrenaline  is 
discharged  when  the  gland  is  stimulated  by  its  nerves. 

Once  in  the  blood,  ad- 
renaline produces  profound 
effects  in  many  organs  of 
the  body.  Among  these  may 
be  mentioned  a  decrease 
in  the  blood  supply  to  the 
digestive  organs;  a  change 
in  the  beat  of  the  heart; 
an  increased  flow  of  blood 
through  the  brain,  the  skele- 
tal muscles,  and,  to  a  less 
extent,  the  skin;  the  dis- 
charge of  sugar  into  the 
blood  by  the  liver;  and  an 
increase  in  the  number  of 
the  red  blood  corpuscles 

FIG.  37.  Diagrams  of  external  (A)  and      (see  P*  136). 

internal  (B)  secretion  It   is   a   most    significant 

fact  that  many  if  not  most 
of  the  reactions  of  the  or- 
ganism to  adrenaline  are  the 
very  reactions  which  are 
needed  in  times  of  great 
muscular  exertion.  For  example,  the  shifting  of  the  blood 
from  digestive  organs  to  the  working  muscles  and  to  the 
brain,  which  is  thereby  rendered  more  alert;  the  supply  of 
increased  quantities  of  sugar  to  serve  as  power  for  muscular 
work ;  the  assistance  to  the  heart,  which  is  called  upon  at  such 
times  to  pump  more  blood;  the  augmented  oxygen-carrying 
capacity  of  the  blood  by  increase  of  its  red  corpuscles  —  all 


The  passage  of  food  material  from  the 
capillaries  into  the  gland  cells  is  repre- 
sented by  the  arrows  with  broken  lines; 
the  path  of  discharge  of  the  secretion,  in  A 
into  the  duct  and  in  B  into  the  blood,  is  in- 
dicated by  the  arrows  with  unbroken  lines 


INTERDEPENDENCE  OF  ORGANS  67 

these  reactions  place  at  the  disposal  of  the  muscles  and  nerv- 
ous system  the  conditions  for  maintaining  intense  work  for 
comparatively  brief  periods  of  time,  and  all  this  is  done  by 
the  simple  expedient  of  discharging  from  one  of  the  organs 
of  the  body  an  internal  secretion  on  the  blood. 

Finally,  that  adrenaline  does  in  fact  serve  the  purpose  of 
placing  the  body  in  condition  to  perform  intense  muscular 
work  is  rendered  probable  by  the  discovery  that  conditions 
of  emotional  excitement,  especially  those  of  fear  or  anger, 
cause  the  discharge  of  nervous  impulses  to  the  adrenals. 
Among  animals  it  is  these  very  emotions  which  accompany 
or  at  least  precede  the  most  vigorous  muscular  activity, 
fear  going  along  with  flight  and  anger  with  combat.  This 
suggests  that  these  emotions  serve  the  purpose  of  calling 
forth  the  utmost  of  which  the  animal  is  capable  in  preserving 
its  very  existence. 

5.  Other  examples  of  internal  secretion.  An  equally  re- 
markable discovery  has  shown  that  the  pancreas  not  only 
manufactures  an  important  digestive  juice  (pancreatic  juice) 
which  it  discharges  into  the  intestine  through  its  duct  (pan- 
creatic duct,  see  Fig.  54)  but  also  produces  another  sub- 
stance which  is  necessary,  in  order  that  other  organs  may 
use  the  sugar  which  is  in  their  food.  Here  we  have  an  ex- 
ample of  an  organ  which  produces  both  an  ordinary  and 
an  internal  secretion,  and  the  same  thing  seems  to  be  true 
of  the  kidney,  as  it  certainly  is  of  the  liver. 

Much  attention  has  recently  been  given  to  the  study  of 
another  ductless  gland,  the  pituitary  body,  situated  in  the 
bone  between  the  roof  of  the  nasal  cavity  and  the  base  of 
the  brain  (see  Fig.  14).  There  is  good  reason  for  thinking 
that  this  gland  contributes  an  important  internal  secretion 
to  the  blood  and  that  certain  organs  of  the  body  fail  to  act 
normally  when  this  secretion  is  deficient ;  serious  results  also 
follow  an  excessive  secretion.  Incidentally  it  may  be  men- 
tioned that  it  is  widely  held  that  excessive  secretion  of  the 


68  THE  HUMAN  MECHANISM 

thyroid  leads  to  a  very  serious  condition,  known  as  Graves's 
disease  or  exophthalmic  goiter,  just  as  deficiency  of  the  secre- 
tion leads  to  the  entirely  different  disease  to  which  we  have 
already  referred. 

Thus,  through  the  medium  of  the  blood  the  chemical 
activity  of  one  organ  may  affect  the  life  of  other  organs 
favorably  or  unfavorably.  All  the  cells  of  the  body  help  to 
make  the  blood  what  it  is,  many  of  them  contributing  to  it 
something  useful  or  even  necessary  to  other  cells.  The  work 
of  the  body  is  not  merely  the  sum  total  of  the  work  of  its 
separate  cells,  each  working  for  itself  alone  and  performing 
a  single  function;  between  the  cells  an  exchange  of  prod- 
ucts often  takes  place,  so  that  cells  become  both  serviceable 
to  and  dependent  upon  one  another  for  the  material  needed 
to  carry  out  their  own  special  chemical  activities.  And  what 
is  true  of  cells  is  no  less  true  of  organs;  these  also  are 
interdependent,  ministering  to  one  another. 


CHAPTER  VII 

THE  ADJUSTMENT  OK,  COORDINATION  OF  THE 
WORK  OF  ORGANS  AND  CELLS 

A  great  physiologist  once  said,  "  Science  is  not  a  body  of 
facts ;  it  is  the  explanation  of  facts."  Some  of  the  most  im- 
portant chapters  of  science  are  those  which  seek  to  explain 
facts  so  well  known  and  obvious  that  we  are  apt  to  forget 
that  they  need  explanation.  When  anything  irritates  the 
lining  of  the  nasal  cavity  we  sneeze ;  when  it  irritates  the 
larynx  we  cough;  when  it  irritates  the  exposed  surface  of 
the  eyeball  we  wink.  These  three  facts  are  well  enough 
known ;  but  it  is  safe  to  say  that  anyone  considering  the 
matter  for  the  first  time  would  find  it  difficult  to  explain 
how  it  comes  about  that  anything  going  "  down  the  wrong 
way "  does  not  make  us  sneeze  or  wink,  but  sets  us  to 
coughing.  The  answer  to  the  general  question  thus  raised 
is  the  subject  of  this  chapter,  which  considers  the  adjust- 
ment of  the  work  of  the  individual  cells  and  organs  of  the 
body,  each  to  do  its  work  at  the  proper  time  and  so  to  play 
its  due  part  in  the  work  of  the  organism  as  a  whole. 

The  more  we  think  of  it,  the  more  wonderful  does  this 
fact  of  adjustment  appear.  The  millions  of  living  cells  are 
in  a  way  individual  units,  and  communities  of  individuals  do 
not  invariably  work  together.  Let  us  compare  the  human 
body  in  this  respect  with  bodies  or  groups  of  men  or  boys. 
In  a  game  of  football  each  team  is  a  body  of  eleven  indi- 
viduals, and  each  individual  is  assigned  to  a  definite  posi- 
tion to  do  definite  things  as  occasion  arises.  Theoretically, 
under  given  conditions  of  the  game  it  is  the  work,  or 
function,  of  the  "  left  tackle "  to  prevent  a  certain  player 


70  THE  HUMAN  MECHANISM 

of  the  opposing  side  from  making  a  certain  play.  But  there 
is  always  a  doubt  whether  he  will  do  this  thing  or  "  lose 
his  head"  and  do  something  else,  leaving  his  man  free  to  do 
what  he  pleases.  In  the  latter  case  that  organism  which  we 
call  a  football  eleven  would  act  very  much  as  the  human 
organism  would  act  if  it  were  to  wink  and  not  cough  when  a 
foreign  body  lodges  on  the  lining  membrane  of  the  larynx. 

Evidently  we  have  something  here  to  explain.  Why  are 
the  actions  of  the  body  purposeful;  that  is,  adapted  to  ac- 
complish the  right  thing  at  the  proper  time?  And  in  the 
more  complicated  actions  how  is  the  work  of  the  different 
units  —  the  organs  and  the  cells  —  adjusted,  or  coordinated', 

that  is  to  say,  how  is  each 
one  made  to  do  its  proper 
share  of  the  work  ?  Let 
us  begin  with  the  study 
of  a  very  simple  action  — 
that  of  winking. 

1.  Winking  is  caused  by 

the  contraction  of  muscle 
FIG.  38.    The  muscular  mechanism        fiberg     which     mn      trang_ 
of  winking 

versely   across    the   eyelid 

in  a  curved  course.  As  they  are  attached  most  firmly  at 
the  regions  A  and  B  (Fig.  38),  their  shortening  straightens 
their  arched  course  and  so  brings  the  two  edges  of  the  eye- 
lid into  contact.  The  work  of  this  muscle  is  obviously  pur- 
poseful, for  the  wink  takes  place  when  the  eyeball  needs 
protection;  it  is  also  coordinated,  since  the  act  is  executed 
by  a  number  of  fibers  working  together,  for  if  only  those  of 
the  lower  eyelid  were  to  contract  the  lids  could  not  be  closed. 
The  muscle  fibers  which  work  together  to  produce  the 
wink  do  not  originate  their  own  activity.  They  merely  do 
what  they  are  stimulated  to  do  by  the  nervous  impulse, 
which  acts  upon  the  muscular  fuel  substance  somewhat  as 
a  fuse  acts  upon  a  charge  of  gunpowder.  Even  the  amount 


COORDINATION 


71 


of  contraction  is  determined  by  the  strength  of  the  nervous 
impulse,  a  strong  impulse  producing  greater  contraction 
than  a  weak  impulse.  In  health  the  muscle  fibers  are  the 
obedient  servants  of  the  nerves,  and  if  they  act  in  a  pur- 
poseful and  coordinated  manner,  it  is  because  the  nerves 
stimulate  them  to  act  in  this  way.  The  explanation  of 
purposeful  and  coordinated  action  must  therefore  be  sought 


FIG. 


Cross  section  of  a  nerve 


Showing  five  bundles  of  nerve  fibers  bound  together  by  connective  tissue  con- 
taining a  few  blood  vessels.  On  the  right  are  shown  four  fibers  more  highly 
magnified,  the  dark  center  being  the  ax  on,  around  which  is  the  white  or  fatty 
sheath,  both  axon  and  fatty  sheath  being  inclosed  within  the  fine  membrane, 
the  neurilemma.  Cf .  Fig.  40 

not  in  the  muscles  but,  behind  these,  in  the  nervous  system, 
to  the  study  of  which  we  now  turn. 

2.  Structure  of  a  nerve.  A  nerve,  like  a  muscle,  may  be 
separated  into  long  fibers  (Fig.  40)  which  are  bound  together 
by  connective  tissue  containing  blood  vessels,  lymph  spaces, 
and  lymphatics.  The  nerve  fiber,  which  is  the  essential 
part  of  the  nerve,  just  as  the  muscle  fiber  is  of  the  muscle, 
differs  somewhat  in  structure  in  different  nerves;  it  generally 


72 


THE  HUMAN  MECHANISM 


consists  of  a  central  threadlike  core  surrounded  by  a  fatty 
sheath,  the  latter  being,  therefore,  shaped  like  a  hollow 
cylinder,  —  which,  however,  is  interrupted  at  intervals  of 
about  one  millimeter,  —  and  both  of  these  are  enveloped  in  a 
delicate  membrane  comparable  to  the  sarcolemma  of  the  muscle 
fiber.  Such  fibers  are  from  about  -5-^5-5-  to  y-^Q-  of  an  inch 
in  diameter  (compare  the  diameter  of  a  muscle  fiber,  p.  34). 

There  are,  however,  nerve  fibers 
which  have  no  fatty  sheath,  and 
others  which  are  destitute  of  mem- 
brane. The  essential  part  of  the 
fiber  is  the  threadlike  portion  in 
the  center;  this  is  never  absent 
from  nerves  and  is  known  as  the 
axon,  or  axis  cylinder. 

3.  The  axon  of  a  nerve  fiber  is 
a  branch  of  a  nerve  cell.  By  suit- 
able methods  these  axons  may  be 
traced  along  the  nerve  of  which 
they  form  part  and  even  into  the 
brain  and  spinal  cord;  it  is  then 
found  that  they  pursue  an  uninter- 
rupted course  and  ultimately  become  continuous  with  the 
cytoplasm  of  a  nerve  cell.  Nerve  cells  are  found  in  the  brain, 
in  the  spinal  cord,  in  enlargements  (ganglia)  on  certain 
nerves,  and  even  alone  in  the  connective  tissue  of  many  or- 
gans of  the  body,  as  the  heart,  the  intestine,  etc.  By  far  the 
greater  number  are  in  the  brain  and  spinal  cord,  and  in  some 
cases  the  axons  to  which  they  give  rise  are  of  very  consider- 
able length;  those  of  the  muscles  of  the  foot,  for  example, 
reach  from  cells  in  the  sacral  region  of  the  spinal  cord  to  the 
extremity  of  the  foot.  Such  fibers  would  be  over  a  yard  long 
and  less  than  y^o"  °^  an  mc^  wide,  an(^  we  mav  regard  the 
cell  whose  main  portion  is  in  the  sacral  cord  as  sending  out 
a  branch,  or  process,  from  this  region  to  the  foot. 


FIG.  40.    Four  nerve  fibers 
(highly  magnified) 

R,  node  of  Ranvier  at  which 
the    fatty    sheath    is    discon- 
tinuous 


COORDINATION  73 

Furthermore,  recent  investigations  have  led  to  the  gener- 
ally accepted  conclusion  that  each  axon  is  a  part  of  only  one 
nerve  cell;  a  single  cell  may  give  off  more  than  one  axon, 
but  the  axon  is  never  connected  with  more  than  one  nerve 
cell.  Of  these  cells  and  of  their  connections  with  nerve 


FIG.  41.   Four  nerve  cells 


A  and  C,  from  the  cerebellum ;    B,  from  the  gray  matter  of  the  spinal  cord ; 

D,  from  the  cerebrum ;  a,  the  axon.    The  cells  A  and  D  are  stained  so  that  the 

main  body  and  the  dendrites  (p.  75)  are  a  uniform  hlack ;  B  and  C  are  stained  so 

as  to  show  the  nucleus  and  the  cytoplasm 

fibers  we  can  get  a  more  definite  picture  by  an  examination 
of  the  structure  of  the  spinal  cord. 

4.  Structure  of  the  spinal  cord.  When  the  vertebral  canal 
is  opened  a  whitish  cord  is  found  within  it,  —  the  spinal 
cord,  —  from  each  side  of  which  arise  thirty-one  pairs  of 
nerves,  or,  in  general,  one  pair  for  each  vertebra.  One 
nerve  of  each  pair  arises  on  the  ventral  side  of  the  cord,  the 
other  on  the  dorsal  side.  These  nerves  are  known  as  the 


74 


THE  HUMAN  MECHANISM 


Dorsal 


Dorsal  Root 


Dorsal 
Ganglion 


ventral  and  dorsal  nerve  roots1  respectively.  On  the  dorsal 
nerve  root,  some  distance  from  the  cord,  there  is  a  slight 
enlargement,  or  ganglion.  Just  outside  this  ganglion  the 
two  roots  unite,  and  from  their  union  nerves  pass  to  the 
skin,  the  muscles,  the  blood  vessels,  the  viscera,  etc. 

The  spinal  cord  itself  in  cross  section  shows  a  darker 
central  core,  known  as  the  gray  matter,  surrounded  by  an 
outer  lighter  portion,  the  white  matter.  The  white  matter 

consists  essentially  of 
nerve  fibers  which  run 
lengthwise  of  the  cord 
and  here  and  there  send 
branches  into  the  gray 
matter;  it  may  be  re- 
garded as  a  large  nerve. 
The  gray  matter,  on  the 
other  hand,  contains  a 
mesh  of  fibers  and,  in  ad- 
dition, numerous  nerve 
cells.  There  is  the  same 
difference  everywhere 
between  the  white  and 
gray  matter  of  the  nerv- 
ous system ;  the  arrange- 
ment in  the  brain  is  not 
so  simple  as  in  the  cord,  but  here  also  the  white  matter  con- 
sists of  fibers  running  from  one  part  of  the  nervous  system 
to  another,  while  the  masses  of  gray  matter  always  include 
collections  of  nerve  cells. 

5.  Fibers  of  the  ventral,  or  anterior,  nerve  root.  These 
fibers  may  be  traced  into  the  spinal  cord.  It  is  then  found 
that  the  nerve  cells  from  which  they  arise  lie  in  the  gray 
matter  in  the  immediate  neighborhood  of  the  root  to  which 

1  The  older  anatomical  terms  and  those  even  to-day  more  generally  used 
are  "anterior"  and  "posterior,"  instead  of  "ventral"  and  "dorsal." 


Dorsal 


Dorsal  Root 

Ganglion 

FIG.  42.     The   origin   of   the   dorsal   and 

ventral  nerve  roots  of  a  segment  of  the 

spinal  cord 


COOKDINATION  75 

they  belong;  that  is,  the  fibers  of  the  roots  do  not  come  from 
higher  or  lower  parts  of  the  cord  or  from  the  brain.  It  has 
been  found  that  when  these  roots  are  stimulated  they  throw 
muscles  into  contraction  and  produce  effects  on  the  blood 
vessels  and  glands,  but  they  do  not  give  rise  to  sensations 
or  produce  other  effects  in  the  cord  itself.  In  other  words, 
the  fibers  of  the  ventral  root  conduct  impulses  from  the  cells  of 
the  spinal  cord  outward;  they  do  not  conduct  impulses  from 
outside  into  the  spinal  cord.  Hence  they  are  known  as 
efferent  fibers  (Latin  ex,  "  out  of  "  ;  ferre,  "  to  carry  "  ). 

The  nerve  cells  from  which  these  fibers  arise  consist  of  a 
mass  of  cytoplasm  around  the  nucleus  and  of  one  or  more 
outgrowths  of  this  cytoplasm,  usually  more  or  less  branched. 
These  outgrowths  of  the  cytoplasm  divide  and  subdivide,  ulti- 
mately forming  in  the  gray  matter  exceedingly  fine  terminal 
branches  like  those  of  a  tree  in  the  air.  Such  processes  are 
known  as  dendrites  (Greek  dendron,  "  a  tree  "  ).  The  nerve 
cells  in  question  have  numerous  dendritic  processes ;  in  other 
nerve  cells  there  may  be  but  one,  and  still  others  possess 
no  den'dritic  processes  at  all.  In  all  cases  the  general  appear- 
ance of  the  cell  depends  largely  upon  the  number  and  man- 
ner of  branching  of  these  dendrites.  Thus  it  happens  that 
nerve  cells  differ  from  one  another  in  appearance  just  as 
a  Lombardy  poplar,  an  oak,  an  elm,  and  a  maple  differ, 
although  all  show  the  fundamental  characteristics  of  a  tree 
(Fig.  41;  see  also  Figs.  109,  110,  and  111). 

In  subsequent  portions  of  this  work  it  is  unnecessary  for 
us  to  go  into  the  details  of  the  form  of  the  nerve  cells  to 
any  extent ;  the  student  need  only  understand  henceforward 
that  nerve  cells  consist  of  a  central  mass  of  nucleated  cyto- 
plasm from  which  proceed  outgrowths,  or  processes,  which 
are  of  two  kinds:  (1)  those  which  become  axons  of  nerve 
fibers  and  which  form  an  essential  part  of  all  nerve  cells ; 
and  (2)  the  dendrites,  which  are  usually  but  not  always 
present.  The  whole  structure,  including  the  central  cell 


76 


THE  HUMAN  MECHANISM 


body  with  its  dendrites  and  axons,  is  an  anatomical  unit  — 
a  cell.  To  this  entire  cell  the  term  "  neurone  "  is  given.  The 
neurone  is  the  cellular  unit  of  the  nervous  system,  just  as 
the  muscle  fiber  is  the  cellular  unit  of  the  muscle,  and  the 
gland  cell  of  the  gland. 

6.  Fibers  of  the  dorsal,  or  posterior,  roots.  The  ventral 
roots,  as  we  have  seen,  are  entirely  efferent  in  function ; 
that  is,  they  conduct  impulses  only  away  from  the  spinal 


FIG.  43.   Semidiagrammatic  longitudinal  section  of  a  ganglion  of  the  dorsal 

(posterior)  root 

cord.  The  dorsal,  or  posterior,  roots,  on  the  other  hand,  are 
found  to  be  essentially  afferent  (Latin  ad,  "  to  " ;  ferre,  "  to 
carry  "  )  ;  that  is,  they  carry  impulses  from  outside  toward  and 
into  the  spinal  cord.  This  is  shown  by  the  fact  that  when 
these  roots  are  destroyed  by  disease,  muscles  can  still  be 
thrown  into  contraction,  glands  will  still  secrete,  etc., — 
that  is,  there  is  no  interference  with  efferent  impulses,  — 
but  no  sensations  are  received  from  the  part  of  the  body 
to  which  these  nerves  are  distributed;  pinching  the  skin  is 
riot  felt;  the  flesh  may  be  burned  and  its  owner  suffer  no 


COORDINATION  77 

pain.  Since  these  results  never  follow  destruction  of  the 
ventral  roots,  we  must  conclude  that  impulses  enter  the  cord 
solely  by  the  dorsal  roots  precisely  as  they  leave  the  cord  solely 
by  the  ventral  roots. 

It  has  been  stated  above  (p.  74)  that  there  is  a  ganglion 
on  the  dorsal  root.  Microscopic  study  of  this  ganglion  shows 
that  the  fibers  of  the  dorsal  root  pass  through  it  and  that 
each  fiber  gives  off  at  right  angles  to  itself  a  branch  which 
becomes  continuous  with  a  pear-shaped  nerve  cell  of  the 
ganglion.  These  cells  have  no  other  processes.  We  may 
express  the  relation  between  the  pear-shaped  cells  of  the 
ganglion  and  the  fibers  of  the  dorsal  root  by  saying  that  the 
single  axon  from  the  main  cell  body  divides  into  two  in 
the  ganglion,  one  branch  passing  outward  to  the  periphery, 
the  other  passing  centrally  into  the  spinal  cord  (Fig.  43). 

7.  Endings  of  the  peripheral  branches  of  the  neurones  of  the 
dorsal  root  in  sense  organs.    The  peripheral  branch  ultimately 
ends  in  some  "  sense  organ,"  one  of  the  most  important  of 
which,  so  far  as  the  spinal  nerves  are  concerned,  is  the  skin. 
The  eye,  the  ear,  the  nose,  the  mouth,  are  examples  of  other 
sense  organs,  and  they  all  contain  the  peripheral  endings  of 
afferent  neurones.    Each  is  sensitive  to  some  special  influence 
from  without,  as  the  eye  to  light,  the  ear  to  sound,  etc. ; 
and  when  stimulated  they  start  nerve  impulses  moving  in- 
ward along  the  nerves  toward  the  brain  or  cord. 

8.  Ending  in  the  spinal  cord  of  the  central  branch  of  the 
neurones  of  the  dorsal  root.    The   other    or   central   branch 
passes  into  the  spinal  cord.    It  does  not,  however,  like  the 
neurones  of  the  ventral  root,  there  become  continuous  with 
the  nerve  cells  of  the  gray  matter,1  but  divides,  on  entering 
the  cord,  into  an  ascending  and  a  descending  branch  (Fig.  44), 
each  of  which  runs  for  a  longer  or  shorter  distance  in  the 
white  matter  of  the  cord.     Indeed,  many  of  the  ascending 

1  It  is,  as  has  already  been  pointed  out  on  this  page,  part  of  a  nerve  cell 
in  the  ganglion  of  the  dorsal  root. 


78 


THE  HUMAN  MECHANISM 


branches  extend  as  far  anteriorly  as  the  lower  parts  of  the 
brain.  As  shown  in  the  figure,  these  branches  give  off  at 
right  angles  to  themselves  subbranches  (the  collaterals),  each 
of  which  enters  the  gray  matter  and  ends  there  by  breaking 
up  into  a  tuft  of  extremely  fine  fibrils,  the  synapse.  The 
synapse  is  in  close  proximity  to,  and  possibly  in  a  kind  of 
anatomical  continuity  with,  the  dendrites  or  the  main  body 


A  B 

FJG.  44.  Relation  of  afferent  (of)  to  efferent  (ef)  neurones  of  the  spinal  cord 

In  A  the  single  afferent  neurone  branches  into  six  collaterals,  each  of  which  ends 

in  a  synapse  around  an  efferent  cell.    In  B  the  connection  is  made  through  the 

agency  of  the  cell  a,  as  explained  in  section  13 

of  a  nerve  cell  of  the  gray  matter.  Each  afferent  neurone, 
then,  is  a  cell  whose  main  body  is  in  the  ganglion  of  the 
dorsal  root  and  whose  branches,  or  arms,  reach  out,  one  of 
them  to  a  peripheral  sense  organ  and  the  other  to  the  gray 
matter  of  the  spinal  cord  and  brain,  where  they  end  in 
synapses.  By  means  of  the  synapses  the  afferent  neurone 
excites  or  stimulates  other  neurones. 

9.  Anatomical  relation  of  afferent  to  efferent  neurones.  We 
may  now  put  together  what  we  have  learned  about  the 
neurones  of  the  ventral  and  those  of  the  dorsal  root;  we 


COORDINATION 


79 


then  obtain  a  plan  like  that  shown  in  Fig.  44,  and  such,  in 
principle  at  least,  represents  the  manner  in  which  the  afferent 
neurone  is  brought  into  relation  with  efferent  neurones. 

Afferent  and  efferent  fibers  enter  and  leave  portions  of  the 
brain  in  much  the  same  way,  although  the  separation  into 
ventral  and  dorsal  roots  is  not  obvious.  We  may  there- 
fore take  the  above  scheme  as  typical  of  the  relation  be- 
tween these  two  kinds  of  neurones  —  those  of  the  brain  as 
well  as  those  of  the  cord. 

10.  Application  of  these  facts 
of  structure  in  the  explanation 
of  purposeful  and  coordinated 
action.  The  diagram  in  Fig.  45 
readily  explains  why  the  sud- 
den appearance  of  an  object  in 
front  of  the  eye  causes  us  to 
wink  and  not  cough ;  that  is  to 
say,  it  explains  the  purposeful 
character  of  this  so-called  reflex 
action.  The  formation  of  the  FIG.  45.  Diagram  of  the  nervous 
image  of  the  object  on  the  mechanism  by  which  a  wink  is  pro- 
duced by  the  sudden  appearance  of 
retina,  a  sense  organ,  starts  an  object  in  front  of  the  eye 

impulses     along    the    fibers     of     r,  afferent  neurone  of  the  optic  nerve ; 

the  afferent  optic  nerve  ;  these    m>  m/>  m">  m'">  efferent  neurones  to 

.  .  the  muscles  of  the  eyelid 

fibers  extend  into  the  brain, 

and  their  synapses  end  around  and  stimulate  those  efferent 
nerve  cells  which  stimulate  the  muscles  of  the  eyelid.  The 
action  is  purposeful  because  the  fibers  of  the  optic  nerve 
end  around  these  cells  and  not  around  those  which,  for 
example,  innervate  *  the  muscles  which  open  the  mouth  or 
flex  the  finger  (Fig.  45). 

Our  diagram  also   gives  the  basis  of  coordination  —  the 
combination  of  the  work  of  different  muscle  fibers  in  orderly 
harmonious  action.    The  system  of  collaterals  on  the  central 
i  That  is,  supply  with  nerve  fibers. 


80 


THE  HUMAN  MECHANISM 


branch  of  the  afferent  neurone  is  obviously  a  mechanism  to 
combine  the  action  of  the  efferent  neurones  in  this  way.  The 
diagram  also  gives  a  clew,  at  least,  to  the  explanation  of 
another  element  of  coordination:  when  two  or  more  muscles 
work  together  to  accomplish  a  given  act,  one  of  the  muscles 
usually  works  harder  than  another;  not  only  must  they  work 
together,  but  the  amount  of  force  exerted  by  each  must  be 
adjusted  to  the  needs  of  the  movement  as  a  whole.  This 

adjustment  is  most 
probably  effected  by 
differences  in  the  con- 
nection of  the  ,  syn- 
apses with  their  cells ; 
thus  those  muscles 
which  contract  most 
forcibly  are  innervated 
by  neurones  whose  den- 
drites  and  main  cell 
body  come  into  more 
intimate  contact  with 
the  synapses  of  the 
afferent  neurone ;  or 
the  number  of  fibrils 
of  the  synapse  may 
be  greater  in  their  case 
than  in  the  others.  These,  however,  are  only  possibilities ;  the 
whole  subject  requires  further  elucidation. 

11.  Definition  of  reflex  action.  An  action  such  as  we  have 
just  been  studying  is  known  as  a  reflex1  action.  By  this  we 
mean  an  action  called  forth  by  the  more  or  less  direct  action  of 
afferent  upon  efferent  neurones  and  without  the  intervention  of 

1  The  word  literally  suggests  the  idea  of  reflection  from  the  afferent  to 
the  efferent  neurones,  as  light  is  reflected  from  a  surface  ;  but  the  student 
has  already  learned  enough  to  understand  that  efferent  impulses  are  something 
more  than  mere  mechanical  reflections,  or  rebounds,  of  afferent  impulses. 


FIG.  46.    Diagram  of  the  nervous  mechanism 

represented  in  Fig.  45,  with  the  addition  of 

the  neuron  6  (see  sect.  12) 


COORDINATION 


81 


the  will.  The  afferent  neurone  may  be  stimulated  by  some 
external  agent,  such  as  light,  heat,  sound,  pressure,  etc.,  or 
by  some  condition  within  the  body  itself,  as  when  diseased 
or  abnormal  conditions  of  the  stomach  or  some  other  organ 
induce  vomiting. 

It  is  a  common  error  to  suppose  that  all  actions  which 
are  not  called  forth  by  the  will  are  reflex.  The  essential 
feature  of  a  true  reflex  is  the  more  or  less  direct  action 
of  the  afferent  impulses 
on  efferent  neurones  and 
not  merely  its  nonvoli- 
tional  character.  There 
are,  in  fact,  involuntary 
actions  in  which  the  ef- 
ferent neurones  are  di- 
rectly stimulated  not  by 
afferent  neurones,  but 
by  the  condition  of  the 
blood  or  in  other  ways. 
Such  actions  are  not  re- 
flex, though  they  may 
be  either  involuntary 
or  unconscious  or  both. 
They  are  known,  in  gen- 
eral, as  automatic  actions,  and  we  shall  meet  examples  of  them 
as  we  proceed  with  the  study  of  the  various  functions  of 
the  body. 

12.  Actions  resulting  from  stimulation  by  the  will.  A  wink 
is  not  always  a  reflex  action.  We  can  wink  "  on  purpose," 
or,  otherwise  expressed,  a  wink  may  be  called  forth  by  the 
will  and  entirely  apart  from  the  sudden  appearance  of  some 
object  in  front  of  the  eye.  Here  the  muscles  of  the  eyelid 
act  in  exactly  the  same  manner  as  in  a  reflex  wink,  which 
means  that  they  are  stimulated  in  the  same  way  by  the  same 
efferent  neurones.  Thus  far  the  mechanism  is  the  same  in 


FIG.  47.  The  nervous  mechanism  shown  in 

Fig.  46,  with  the  addition  of  the  afferent 

neurone  c,  from  the  cornea  (see  sect.  12) 


82  THE  HUMAN  MECHANISM 

the  two  cases,  but  the  source  of  stimulation  of  the  efferent 
neurones  must  be  different. 

In  later  chapters  of  this  book  we  shall  bring  forward 
evidence  to  show  that  the  exercise  of  the  will  (volition) 
requires  the  cooperation  of  the  highest  portion  of  the  brain 
or  cerebrum.  Nerve  cells  in  the  gray  matter  of  the  cerebrum 
send  off  axons  whieh  pass  downward  to  those  portions  of  the 
brain  and  spinal  cord  from  which  the  motor  or  efferent  neu- 
rones arise ;  with  the  neurones  of  these  nerves  they  make  ex- 
actly the  same  kind  of  connections  (collaterals  and  synapses) 
as  are  made  by  the  afferent  fibers  from  the  retina  which  excite 
the  reflex  (see  Fig.  46,  in  which  b  is  the  cerebral  neurone). 

The  collaterals  and  synapses  of  the  cerebral  neurone 
(which,  it  will  be  observed,  is  entirely  confined  to  the  cen- 
tral nervous  system)  simply  duplicate  those  of  the  afferent 
neurone ;  hence  the  two  neurones  produce  the  same  result. 

There  is,  however,  still  a  third  way  in  which  winking  may 
be  stimulated.  When  the  cornea  of  the  eye  begins  to  dry,  a 
reflex  wink  spreads  tears  over  the  eyeball.  In  this  case  we 
have  to  deal  with  a  second  reflex,  the  afferent  neurones  being 
not  those  in  the  optic  nerve,  but  those  in  what  is  known  as 
the  trigeminal,  the  sensory  nerve  of  the  cornea.  Our  scheme 
thus  becomes  that  shown  in  Fig.  47. 

13.  The  "  master  "  neurone.  The  multiplication  of  collat- 
erals and  arborizations  which  this  scheme  involves  would 
seem  to  be  largely  avoided  by  the  presence  of  a  third  neu- 
rone between  those  which  stimulate  the  action  and  the  effer- 
ent neurones  which  directly  act  on  the  muscles  (Fig.  48). 

In  this  way,  when  a  wink  is  produced,  whether  from  the 
cerebrum  or  from  the  retina  or  from  the  cornea,  the  single 
cell  a  is  stimulated,  and  this  in  turn  stimulates  the  groups 
of  efferent  neurones  which  immediately  innervate  the  mus- 
cles of  the  eyelids.  Many  of  the  nerve  fibers  of  the  cord  and 
brain  belong  to  neurones  which  perform  the  same  function 
as  that  attributed  to  the  cell  a  in  our  diagram.  They  are 


COORDINATION 


83 


entirely  confined  to  the  brain  or  cord  and  group  together 
those  efferent  cells  which  by  working  together  produce  a 
coordinated  action. 

The  organization  of  the  nervous  system  is,  in  fact,  much 
like  that  of  a  large  manufacturing  establishment.  The  nerve 
cells  which  send  axons  to  the  muscles,  glands,  blood  vessels, 
etc.  may  be  compared  with  the  operatives,  each  with  his 
special  task  to  perform ;  over  these  are  foremen,  or  "  bosses," 
from  whom  they  take  their  orders  or,  in  physiological 
language,  who  stimulate 
them  to  do  their  work 
and  who  would  corre- 
spond to  cells  like  a  in 
Fig.  48.  The  foremen  in 
turn  receive  orders,  now 
from  one  department  of 
the  establishment,  now 
from  another,  as  the  work 
of  their  operatives  is 
needed  in  making  one  or 
the  other  of  the  products 
offered  for  sale.  So  the 
master  neurones  receive 
stimuli  from  the  brain  or  from  afferent  nerves,  as  the  needs 
or  the  desires  of  the  organism  as  a  whole  require  their 
activity.  The  comparison  is  instructive  and  may  easily  be 
carried  out  in  greater  detail  by  the  student  himself. 

14.  The  coordination  of  two  or  more  actions  to  achieve  a 
definite  end.  These  conceptions  will  become  more  definite  if 
we  study  the  nervous  mechanisms  represented  in  Fig.  49, 
which  represents  the  combination  of  the  wink  with  different 
physiological  actions,  according  to  the  nature  of  the  con- 
ditions which  call  it  forth.  Let  us  consider  the  two  reflex 
winks  already  referred  to,  that  from  the  cornea  and  that  from 
the  retina.  The  wink  from  the  cornea  is  for  the  purpose  of 


FIG.  48.    The  master  neurone 


84 


THE  HUMAN  MECHANISM 


spreading  tears  over  the  surface  of  the  eyeball  and,  to  be 
effective,  must  be  accompanied  by  a  secretion  of  tears.  We 
may  suppose  that  this  is  accomplished,  as  in  the  diagram, 
by  the  afferent  neurone  (c)  from  the  cornea  stimulating  two 

master  neurones,  one  of 
which  (mw)  produces  the 
wink,  while  the  other 
(nt)  stimulates  the  tear 
glands  to  secrete. 

The  wink  from  the 
retina,  on  the  other  hand, 
has  the  entirely  different 
purpose  of  preventing 
the  contact  of  foreign 
objects  with  the  cornea. 
For  this  purpose  tears 
are  not  necessary  and 
they  are  not  secreted. 
But  at  times  this  wink 
is  accompanied  by  a 
sudden  starting  back  of 
the  body  as  a  whole 
to  avoid  the  threatened 
danger.  In  this  case  we 
may  suppose  that  the 
afferent  neurone  from 
the  retina  connects  with 
the  master  neurones  mw, 
for  winking,  and  msb,  for 
starting  back,  but  that 
this  afferent  neurone 
does  not  connect  with  m\  for  the  secretion  of  tears. 

Finally,  the  volitional  neurones  vsb  and  vw,  which  pass  from 
the  cerebrum  to  their  appropriate  master  neurones,  call  forth 
these  actions  of  starting  back  or  winking  as  separate  acts. 


FIG.  49.  Coordinations  involved  in  the  com- 
bination of  the  wink  with  other  actions 

The  afferent  neurones  r,  from  the  retina,  and 
c,  from  the  cornea,  connect  with  different  com- 
binations of  efferent  neurones  as  explained  in 
the  text.  Efferent  master  neurones  are  shown 
as  follows :  mw,  for  winking ;  mf,  for  secretion 
of  tears ;  m'b,  for  starting  back.  Neurones  con- 
cerned in  volitional  actions  are  v*b,  for  starting 
back,  and  vw,  for  winking 


COORDINATION  85 

15.  The  acquisition  of  reflexes ;  conditioned  and  uncondi- 
tioned reflexes.  There  can  be  no  doubt  that  many  of  these 
reflex  mechanisms  are  born  with  us.  A  newborn  baby,  for 
example,  like  the  adult,  winks  and  secretes  tears  when  the 
cornea  dries ;  it  secretes  saliva  when  a  sapid  substance  is 
placed  in  the  mouth ;  it  swallows  when  something  touches 
the  throat;  if  a  cane  is  brought  in  contact  with  the  palm 
of  the  hand,  it  is  grasped  firmly.  These  and  many  other 
reflex  actions  take  place  from  the  first  because  the  baby  in- 
herits and  hence  is  born  with  the  complete  reflex  mechanism 
for  their  execution  upon  the  application  of  the  appropriate 
stimuli. 

On  the  other  hand,  new  involuntary  reactions  can  be 
acquired  in  adult  life,  even  reactions  which  are  useless  to 
the  body.  The  extent  to  which  this  is  true  is  illustrated  by 
the  following  extreme  case :  if  a  piece  of  ice  is  applied  to  a 
definite  spot  of  the  skin,  the  amount  of  blood  flowing  through 
that  part  of  the  skin  is  greatly  diminished  and  the  skin  be- 
comes pale.  This  is  an  inherited  reflex  which  (Chap.  XII) 
protects  the  body  from  exposure  to  cold.  A  morsel  of  food 
placed  on  the  tongue  (where  it  stimulates  the  afferent  nerves 
of  taste)  will  reflexly  excite  the  flow  of  saliva.  In  both  cases 
we  see  the  obvious  purposeful  relation  between  the  stimulus 
and  the  reaction  and  in  both  cases  we  are  dealing  with  in- 
herited reflexes.  Moreover,  these  two  reflex  mechanisms  as 
inherited  are  entirely  independent  of  each  other,  for  the 
stimulation  of  the  skin  by  ice  does  not  excite  the  flow  of 
saliva  nor  does  the  stimulation  of  the  sense  of  taste  influence 
the  blood  flow  through  the  skin.  If,  however,  every  time 
that  one  eats,  a  piece  of  ice  is  applied  to  the  same  region  of 
skin,  so  that  loth  these  reflexes  are  simultaneously  excited,  in 
the  course  of  two  weeks  or  more  it  will  be  found  that  the 
application  of  ice  *  to  the  skin  excites  a  flow  of  saliva  even 
though  no  food  is  taken  into  the  mouth.  In  other  words,  these 
two  reflex  mechanisms  have  become  associated,  so  that  activity 


THE  HUMAN  MECHANISM 


of  the  one  now  discharges  the  other.  Evidently  some  sort 
of  nervous  connection  has  been  established  between  them. 
Fig.  50  gives  a  diagram  of  the  new  association  which  has 

been  established 
between  the  two 
centers. 

The  connec- 
tion thus  newly 
established  be- 
tween the  affer- 
ent neurones  of 
cold  (c)  and  the 
efferent  neu- 
rones to  the  sal- 
ivary glands  (s) 
differs  in  sev- 
eral ways  from 
the  connection 
between  the  af- 
ferent and  ef- 
ferent sides  of 
an  inherited 
nervous  mech- 
anism. Such  ac- 
quired reactions 
are  not  evoked 
with  the  same 
certainty  as  the 

inherited  and,  once  acquired,  they  are  more  readily  lost  by 
disuse.  Whether  we  get  the  reaction  or  not  depends  upon 
the  condition  of  the  body  at  the  time  we  apply  the  stimulus. 
Hence  they  are  spoken  of  as  conditioned  reflexes,  to  distinguish 
them  from  the  unconditioned  (or  inherited)  reflexes.  Undoubt- 
edly many  of  our  involuntary  actions,  especially  acquired 
habits  in  general,  are  conditioned  reflexes  acquired  since  birth 


FIG.  50.    The  acquisition  of  a  conditioned  reflex 

A,  reflex  mechanism  for  constriction  of  cutaneous  vessels 
when  cold  is  applied  to  the  skin;  B,  reflex  mechanism  of 
the  secretion  of  saliva  when  a  sapid  suhstance  comes  in 
contact  with  the  tongue.  Above  is  shown  the  usual  normal 
condition  with  no  connection  between  the  two  mechanisms ; 
below,  the  condition  after  both  have  been  repeatedly  in 
simultaneous  action 


COORDINATION  87 

and  thus  added  on  to  the  stock  of  inherited  reflexes  which 
make  part  of  the  equipment  with  which  we  begin  life. 

16.  The  complexity  of  the  mechanisms  of  the  nervous 
system.  Such  actions  as  we  have  been  studying  —  whether 
the  inherited  reflex  of  winking,  even  when  this  is  combined 
with  other  acts  like  the  secretion  of  tears,  or  the  acquired 
conditioned  reflex  secretion  of  saliva  from  the  stimulation 
of  the  skin  by  cold  —  are  comparatively  simple,  as  compared 
with  many  other  actions  of  daily  life,  such,  for  example, 
as  the  throwing  of  a  stone.  Here  not  only  muscles  which 
produce  motion  at  the  shoulder,  elbow,  wrist,  and  finger 
joints  are  called  into  play,  but  also  muscles  which  maintain 
the  erect  position  and  balance  of  the  body  as  a  whole. 
The  entire  nervous  mechanism  involved  baffles  the  imagi- 
nation to  conceive ;  and  yet  any  boy  can  perform  the  act. 
He  can  do  it,  however,  because  his  motor  neurones  are 
grouped  together  into  a  perfectly  well-organized  army  which 
executes  at  once  the  bidding  of  its  commander  in  chief  — 
the  will. 

We  have  given  in  the  foregoing  pages  a  mere  glimpse 
into  the  complexity  of  one  part  of  the  wonderful  nervous 
mechanism.  No  watch,  no  machine  which  man  has  ever 
invented  or  constructed  can  for  a  moment  compare  with  this 
living  machine  in  complexity  or  in  perfection.  Yet,  like  all 
machines,  this  one  can  be  abused ;  it  can  get  out  of  order ;  it 
can  even  break  down.  And  we  have  already  learned  enough 
to  understand  why  this  is  so.  Some  neurones  may  be  injured 
by  overwork  or  may  degenerate  from  disuse ;  indulgence  in 
stimulants  or  narcotics  may  poison  the  governing  nerve  cells ; 
above  all,  constant  failure  to  lead  a  normal  life  may  deprive 
these  cells  of  their  sole  means  of  repair.  The  human  body 
is  a  machine  designed  for  use,  even  for  hard  use,  and  it 
thrives  upon  right  use ;  but  it  is  a  machine  too  delicate  and 
too  complex  to  be  abused  with  impunity. 


88  THE  HUMAN  MECHANISM 

When  one  thinks  of  the  hundreds,  perhaps  thousands,  of 
movements  which  the  body  makes,  and  of  the  combination 
of  these  movements  into  definite  actions  or  work,  and  then 
reflects  that  the  muscle  fibers  which  execute  any  movement 
are  thrown  into  orderly  contraction  by  nerve  cells  which  are 
themselves  commanded  by  higher  nerve  cells;  that  these  in 
turn  are  marshaled,  as  it  were,  by  still  higher  cells  when 
the  separate  movements  they  evoke  are  to  be  combined  into 
a  still  more  complicated  action  —  one  begins  to  appreciate 
the  complexity  of  the  organization  of  the  nervous  system. 
The  number  of  the  nerve  cells  is  measured  by  hundreds  of 
thousands,  and  their  efficiency  in  directing  the  working 
organs  of  the  body,  so  as  to  meet  the  demands  of  life,  de- 
pends not  only  upon  the  integrity  of  the  neurones  but  also 
upon  the  perfection  of  their  organization,  that  is,  their 
grouping  into  squads,  companies,  regiments,  brigades,  divi- 
sions, and  corps,  ready  to  yield  instant  and  obedient  response 
to  the  command  of  the  higher  officers  of  the  will  or  to  the 
signals  of  those  pickets  —  the  sense  organs  and  their  afferent 
neurones  —  which  everywhere  guard  the  outposts  and  give 
information  of  the  need  for  action. 

Moreover,  this  army  of  neurones,  like  any  other  army, 
becomes  efficient  by  work,  by  drilling,  by  practice,  even  by 
battle.  Like  the  soldiers  of  a  regular  army  the  neurones 
may  be  overworked  and  their  efficiency  as  a  military  body 
may  suffer  thereby,  but  they  may  also  work  too  little ; 
the  perfection  of  their  development  and  of  then-  organi- 
zation depends  on  the  practice  they  get  with  reasonable 
activity.  To  this  point  we  shall  return ;  but  meantime 
the  student  can  safely  make  the  application  for  himself. 
Such  comparison  and  such  application  are  not  only  in- 
structive  but  intensely  practical  in  their  bearing  upon 
the  affairs  of  everyday  life  —  upon  that  right  conduct  of 
life  which  is  the  first  duty  of  every  man,  svery  woman, 
every  child.  . 


COORDINATION  89 

17.  Stimulation  and  coordination  by  chemical  means ;  hor- 
mones. In  previous  chapters  we  have  dealt  chiefly  with 
examples  of  stimulation  of  muscle  and  gland  cells  by  nervous 
impulses  and  of  the  coordination  of  the  work  of  organs 
through  the  central  nervous  system;  but  there  is  another 
way  by  which  both  stimulation  and  coordination  are  effected. 
An  irritable  cell  will  respond  to  other  stimuli  than  nervous 
impulses ;  among  these  are  a  sharp  blow,  sudden  heating, 
make  or  break  of  an  electric  current,  and  exposure  to  the 
action  of  certain  substances.  The  last  is  generally  spoken 
of  as  chemical  stimulation,  and  we  shall  meet  with  examples 
of  this  in  our  subsequent  study.  One  will  suffice  for  the 
present.  After  the  food  has  undergone  a  preliminary  diges- 
tion in  the  stomach  by  the  acid  gastric  juice,  it  is  passed 
into  the  small  intestine,  where  its  digestion  is  completed. 
The  first  requisite  for  this  purpose  is  the  secretion  of  pan- 
creatic juice,  and  this  is  secured  as  follows:  the  acid  of  the 
stomach  contents  liberates  from  the  lining  cells  of  the  first 
part  of  the  intestine  a  substance  known  as  secretin,  which 
enters  the  blood  and  chemically  excites  the  cells  of  the  pan- 
creas to  secrete  pancreatic  juice.  By  this  means  the  pancre- 
atic juice  is  secreted  into  the  intestine  at  precisely  the  time 
that  it  is  needed  there ;  that  is,  as  each  consignment  of  acid 
food  is  discharged  from  the  stomach  (see  Chap.  VIII,  p.  113). 
A  substance  thus  liberated  in  one  organ  and  stimulating 
another  organ  to  activity  at  the  time  when  such  activity  is 
needed  is  known  as  a  hormone  (Greek  hormao,  "  I  arouse  "  ). 

The  action  of  secretin  evidently  presents,  in  addition  to  its 
feature  of  stimulation,  an  element  of  purposeful  coordination, 
since  it  insures  the  proper  cooperation  of  the  stomach  and 
pancreas  in  the  work  of  digestion ;  and  other  examples  of  the 
same  thing  might  be  cited.  We  have,  however,  only  to  refer 
the  student  to  the  case  of  adrenaline,  already  described  in 
Chapter  VI,  for  the  most  striking  example  of  coordination 
produced  by  chemical  means. 


90  THE  HUMAN  MECHANISM 

•Cooperation,  adjustment,  and  coordination  are  thus  brought 
about  in  the  body  by  two  means :  first,  through  the  chemical 
action  of  hormones ;  and,  second,  through  the  mechanisms  of 
the  central  nervous  system.  The  first  provides  for  situations 
where  no  great  delicacy  of  adjustment  is  required;  in  the 
secretion  of  the  pancreatic  juice,  for  example,  it  is  not  neces- 
sary that  a  definite  quantity,  no  more  and  no  less,  be  secreted ; 
in  such  a  muscular  movement  as  writing,  on  the  other  hand, 
it  is  necessary  that  each  muscle  taking  part  shall  act  in  a  very 
exact  manner.  For  such  coordinations  the  action  of  the  nerv- 
ous system  is  generally  necessary.  Finally,  as  suggested  by 
our  consideration  of  the  conditioned  reflex,  the  nervous  system 
is  the  chief  means  whereby  we  can  acquire  new  mechanisms 
of  coordination,  thereby  increasing  our  power  of  adjustment 
to  new  conditions  of  life. 


CHAPTER  VIII 
ALIMENTATION  AND  DIGESTION 

A.  THE  SUPPLY  OF  MATTER  AND  POWEK  TO  THE 
HUMAN  MACHINE 

1.  Power  and  the  materials  for  repair  supplied  separately 
to  lifeless  machines.    Living  and  lifeless  machines  are  alike 
in   that  worn-out  parts  must  be  renewed   and  that  power 
must  be  supplied  to  do  work.    In  the  lifeless  machine  these 
two  requirements   are   supplied   separately.    A  factory   and 
its  equipment  of  machinery  are  kept  in  repair  and  enlarged 
(grow)  by  means  of  bricks,  lumber,  steel,  belting,  new  pieces 
of   machinery,   etc.,   which   are    brought    into    the    building, 
while  the  power  which  runs  the  machinery  comes  in  quite 
separately  as  fuel,  or  water  power,  or  electric  power. 

2.  Power  and  the  materials  for  growth  and  repair  supplied 
to  the  human  machine  in  the  one  form  of  foods.    With  the 
human  mechanism  this  is  not  so.    Materials  for  growth  and 
repair,  and  power  for  running,  are  introduced  from  without 
not  separately,  but  together,  both  being  supplied  in  the  one 
form  of  food.    As  it  does  its  life  work  the  human  mechanism, 
like  a  lifeless  machine,  not  only  consumes  power  but  its  parts 
deteriorate,  and  it  is  the  double  function  of  the  food  we  eat 
to  make  good  this  double  loss.    Some  foods  possibly  serve 
only  as  means  of  power;  others  merely  make  good  the  loss 
of  essential  parts  of  the  mechanism;  while  still  others  may 
serve  both  purposes. 

3.  Food  as  a  source  of  power.    Experiment  and  experience 
alike  prove  that  foods  are  the  source  of  power  for  work. 
Bread,  butter,  starch,  sugar,  beef,  and  the  like  may  be  dried 

91 


92  THE  HUMAN  MECHANISM 

and  then  burned  as  fuel,  giving  power  to  an  engine.  The 
occasional  use  of  Indian  corn  or  wheat  for  fuel,  in  the  West, 
the  employment  of  hams  and  bacon  as  fuel  by  steamers  short 
of  coal,  the  explosion  of  flour  dust  in  mills,  and  similar  phe- 
nomena further  illustrate  by  the  teachings  of  experience  the 
fact  that  these  foods  are  rich  in  energy,  or  power. 

When  we  say  that  the  food  must  supply  power  to  the 
body,  we  mean  that  the  power  which  it  contains  must  be 
available  to  the  body.  A  lump  of  coal  may  be  a  source  of 
power,  as  is  shown  by  its  use  in  a  locomotive ;  but  a  lump 
of  coal  would  be  of  no  use  as  food,  because  the  body  has 
no  such  means  of  burning  it  as  has  the  engine.  Again, 
nitroglycerin  contains  chemical  elements  needed  in  the  food; 
but  although  when  exploded  in  a  dynamite  cartridge  it 
may  furnish  power  enough  to  shatter  heavy  armor  plate, 
its  energy  is  not  available  to  the  body. 

Thus,  to  recapitulate,  (a)  food  makes  good  the  loss  of 
living  substance  in  the  body;  (&)  it  supplies  material  for 
growth  and  for  the  manufacture  of  the  secretions  of  the 
body ;  and  (c?)  it  supplies  power  for  the  work  which  the  body 
is  to  do.  It  also  performs  one  more  important  function, 
which  will  be  more  clearly  understood  hereafter;  for  (d)  by 
its  oxidation  food  provides  the  heat  usually  required  to  keep 
up  the  body  temperature.  The  detailed  consideration  of  this 
subject,  however,  must  be  postponed  to  Chapter  XII. 

4.  Chemical  composition  of  foods ;  nutrients.  The  human 
race  has  learned  by  long  experience  that  certain  things  meet 
the  demands  of  the  body  for  food,  and  that  other  things 
do  not.  Perhaps  no  animal  uses  so  many  different  materials 
as  man  in  satisfying  sensations  of  hunger  and  thirst.  Some 
foods  are  taken  from  the  animal  and  some  from  the  vegetable 
kingdom,  and  their  variety  is  greatly  increased  by  special 
modes  of  preparation.  But  however  numerous  the  foods 
from  which  we  prepare  the  dishes  served  at  different  meals, 
chemical  analysis  shows  that  the  essential  constituents  of  all 


ALIMENTATION  AND  DIGESTION  93 

foods  belong  to  a  comparatively  small  number  of  chemical 
groups.  These  classes,  or  groups,  may  be  called  nutrients; 
and  as  all  the  members  of  the  same  group  undergo  practi- 
cally the  same  processes  of  digestion  and  perform  similar 
functions  in  nourishing  the  body,  it  will  be  equally  accurate 
and  more  convenient,  in  treating  of  this  part  of  physiology, 
to  speak  of  the  different  nutrients,  and  not  of  beef,  mutton, 
fish,  eggs,  bread,  milk,  butter,  etc. 

From  the  point  of  view  of  digestion  the  most  important 
nutrients  are  the  proteins,  the  carbohydrates,  the  fats,  the 
inorganic  salts,  and  water-,  and  the  student  must  at  this 
point  become  thoroughly  familiar  with  what  is  meant  by 
these  fundamental  terms. 

5.  The  group  of  proteins.  We  may  obtain  a  working  idea 
of  what  a  protein  is  by  recalling  some  of  the  foods  in  which 
protein  preponderates  or  is  easily  seen.  Such  foods  are  the 
white  of  egg,  the  lean  of  tender  meat  (muscle  fibers),  the 
curd  of  milk,  the  tenacious  gluten  of  wheat.  Proteins  also 
exist  in  relatively  large  quantities,  though  not  so  readily 
seen,  in  yolk  of  egg,  beans,  peas,  oats,  and  other  grains. 

Proteins  contain  carbon,  hydrogen,  nitrogen,  oxygen,  and 
sulphur.  Some  contain  phosphorus  and  some  contain  iron. 
Chemically  they  are  exceedingly  complex  substances.  It 
should  be  noted  that  the  proteins  are  the  most  important 
nutrients  which  contain  nitrogen  and  sulphur. 

Many  proteins  readily  become  insoluble.  Examples  of  this 
are  the  hardening  of  the  white  of  egg  or  the  lean  of  meat 
by  cooking  and  of  the  casein  or  curd  of  milk  by  rennet  or 
"  junket  tablets."  This  change  is  known  as  coagulation,  and 
most  of  our  protein  food  is  eaten  after  having  been  coagulated 
in  the  process  of  cooking. 

Proteins  occur  only  within  the  living  cells  of  plants  and 
animals  or  as  the  products  of  these  living  cells.  They  form, 
as  we  shall  more  clearly  see  later,  an  essential  part  of  the 
basis  of  the  living  cell  and  are  constantly  disintegrating 


94  THE  HUMAN  MECHANISM 

within  the  cell  into  simpler  substances.  Hence  there  is  a 
constant  cellular  loss  of  protein,  which  in  the  animal  body 
can  be  made  good  only  from  protein  in  the  food.  Plants,  on 
the  other  hand,  have  the  power  of  manufacturing  proteins 
from  sugars  and  certain  mineral  salts,  the  latter  supplying 
the  needed  nitrogen  and  sulphur.  The  plant  kingdom  is, 
therefore,  in  the  long  run  the  sole  source  of  protein  food  for 
animals ;  for  while  some  animals  (carnivores)  get  their  pro- 
tein entirely  by  eating  the  flesh  of  other  animals,  the  latter 
(herbivorous  animals)  in  turn  have  obtained  their  protein 
from  plants. 

Unlike  fats  and  carbohydrates,  protein  is  an  absolute 
essential  of  animal  diet;  that  is  to  say,  protein  food  per- 
forms certain  functions  in  the  animal  body  which  cannot  be 
performed  by  fats  or  carbohydrates,  while  the  two  latter 
nutrients  perform  no  functions  which  cannot  also,  when 
necessary,  be  met  by  proteins.  Some  proteins,  however,  are 
incapable  of  meeting  all  the  protein  requirements  of  the 
organism,  although  they  may  meet  some  of  them.  Of  these 
the  most  important  in  use  as  food  is  the  fibrous  connective 
tissue  (pp.  7,  8),  whose  fibers  in  the  uncooked  state  consist 
of  the  insoluble  protein  substance  collagen,  which  by  heating 
in  the  presence  of  water  is  converted  into  the  closely  related 
but  soluble  gelatin.  Collagen  and  gelatin  belong  to  the 
albuminoids,  one  of  the  subclasses  of  proteins.  The  chief 
protein  of  Indian  corn  is  similarly  incapable  of  meeting  all 
the  protein  requirements  of  the  organism. 

6.  The  group  of  carbohydrates ;  the  plant  cell  as  a  food 
factory.  The  carbohydrates  constitute  a  very  large  chemical 
group,  although  comparatively  few  members  of  it  (starch 
and  sugars)  are  of  importance  as  food.  They  are  all  com- 
pounds of  the  elements  carbon,  hydrogen,  and  oxygen,  and 
contain  no  nitrogen  or  sulphur;  those  used  as  food  are 
manufactured  in  the  cells  of  green  plants.  This  production 
of  carbohydrates  by  the  plant  cell  is  another  example  of  the 


ALIMENTATION  AND  DIGESTION  95 

work  of  cells  as  chemical  factories,  which  we  studied  in 
Chapter  IV.  The  cells  of  the  green  parts  of  plants,  espe- 
cially of  the  leaves,  take  in  carbon  dioxide  from  the  air  and 
water  from  the  soil,  and  from  these  plant  foods,  with  the 
aid  of  sunlight,  manufacture  sugar,  which  is  transported  in 
the  sap  from  one  part  of  the  plant  to  the  other  and  is  used 
as  a  source  of  power  for  plant  work.  The  excess  of  sugar 
is  converted  by  certain  cells  into  starch  and  is  stored  in  the 
form  of  small  granules  in  the  cytoplasm  for  future  use. 
A  potato  or  a  grain  of  wheat  consists  of  cells  loaded  with 
these  starch  granules.  When  the  plant  is  not  manufacturing 
sugar  directly  from  carbon  dioxide  and  water,  its  cells  again 
transform  the  starch  granules  into  sugar.  The  presence  of 
sugar  in  sugar  beets,  apples,  pears,  and  peaches  and  in  the  sap 
of  sugar  maples  are  familiar  examples  of  this  manufacture 
and  transport  of  sugar  by  plants. 

It  will  be  noticed  that  only  green  plants  have  this  power 
of  manufacturing  carbohydrates  from  carbon  dioxide  and 
water;  hence  we  do  not  find  large  quantities  of  sugar  and 
starch  in  mushrooms  and  other  fungi.  The  cells  of  green 
plants,  in  short,  are  the  starch  factories  of  the  world,  the 
factories  from  which  we  purchase  our  supplies  of  starch  be- 
ing only  refineries,  that  is,  places  where  starch  is  separated 
from  other  constituents  of  plant  cells. 

All  plants,  however,  possess  the  power  of  manufacturing 
proteins  from  carbohydrates  and  certain  salts,  which  salts 
they  get  from  the  soil.  The  carbohydrates  furnish  carbon, 
hydrogen,  and  some  of  the  oxygen,  while  the  salts  furnish 
nitrogen,  sulphur,  phosphorus,  etc.  One  great  difference 
between  plants  and  animals  is  this  power  of  protein  manu- 
facture by  the  cells  from  material  which  is  not  protein.  The 
animal  cell  can  manufacture  protein  only  from  protein  itself 
or  from  certain  decomposition  products  of  protein. 

7.  The  group  of  fats.  Fats  are  familiar  to  us  in  such 
forms  as  butter,  lard,  olive  oil,  and  the  fat  of  meat.  Like 


96  THE  HUMAN  MECHANISM 

the  carbohydrates  they  are  compounds  of  carbon,  hydrogen, 
and  oxygen,  although  the  oxygen  is  always  present  in  small 
quantities.  The  formula  for  one  of  the  fats  is  C51H98O6,  and 
this  composition  is  typical  of  all  of  them. 

Fats  may  be  split  up  into  certain  acids  (fatty  acids)  and 
glycerin,  and  when  treated  with  alkalies,  like  caustic  soda  or 
caustic  potash,  they  form  soaps.  They  are  insoluble  in  water. 
Like  the  carbohydrates  they  contain  no  nitrogen. 

8.  Oxidizable  and  nonoxidizable  nutrients.    All  the  above 
nutrients  may  and  do  combine  with  oxygen  within  the  cells 
of  the  body,  although  the  way  in  which  this  chemical  union  is 
brought  about  is  one  of  the  unsolved  problems  of  physiology. 
While  all  the  nutrients  may  be  burned  after  being  dried, 
such  combustion  requires  a  high  temperature.     Within  the 
body  they  are  not  only  burned  (that  is,  combined  with  oxy- 
gen) at  a  temperature  rarely  exceeding  39°  C.  (100°  F.),  but 
they  undergo  oxidation  while  in  a  moist  state  or  even  in 
solution.    However  this  oxidation  may  be  effected  within  the 
cell,  there  can  be  no  doubt  that  it  yields  the  heat  for  keeping 
the  body  warm  and  possibly  the  power  for  its  work. 

The  remaining  groups  of  nutrients  —  the  inorganic  salts 
and  water  —  are,  for  the  most  part,  not  oxidized  in  the  body. 

9.  The  groups  of  inorganic  salts  and  water.    These  nutrients 
are  absolutely  necessary  for  the  proper  nourishment  of  the 
body,  their  presence  in  the  blood  and  lymph  and  in  the  liv- 
ing cells  being  indispensable  to  the  processes  of  life.    The 
salts  are  taken  in  small  quantities,  partly  as  salt  itself,  partly 
as  portions  of  the  various  foods  we  eat.    During  growth  they 
furnish  much  of  the  mineral  matter  of  bones,  and  since  the 
body  is  daily  losing  salt,  it  is  necessary  that  salt  be  supplied 
in  the  food.    Salts,  however,  are  not  acted  on  to  any  large 
extent  in  the  alimentary  canal  by  the  processes  of  digestion; 
they  are  largely  absorbed  in  the  same  form  as  eaten.    Hence 
they  do  not  concern  us  at  present  to  the  same  extent  as  do  the 
oxidizable  nutrients,  which  generally  have  to  be  chemically 


ALIMENTATION  AND  DIGESTION 


97 


changed,  or  digested,  before  they  can  be  absorbed  for  use  in 
the  body.    The  same  thing  is  true  of  water. 

10.  Composition  of  some  common  foods.  The  following  table 
gives  the  percentage  composition  of  some  of  the  more  common 
foods  (see  also  p.  238). 


WATER 

PROTEIN 

STARCH 

SUGAR 

FAT 

SALTS 

Bread         .    . 

37 

8 

47 

3 

1 

2 

Wheat  flour  .... 
Oatmeal     

15 
15 

11 
12.6 

66 

58 

4.2 
5  4 

2 
5.6 

1.7 
3 

Rice 

13 

6 

79 

04 

0  7 

0  5 

Peas  

15 

23 

55 

2 

2 

2 

Potatoes     

75 

2 

18 

3 

0  2 

0  7 

Milk 

86 

4 

5 

4 

0  8 

Cheese   

37 

33 

24 

5 

Lean  beef  .  ''.    .    .    v 

72 

19 

3 

1 

Fat  beef 

51 

14 

29 

1 

72 

18 

5 

1 

Veal           .        .    . 

63 

16 

16 

1 

White  fish  

78 

18 

3 

1 

Salmon  ...        .    . 

77 

16 

5  5 

1.5 

Eo-ff 

74 

14 

10  5 

1  5 

Butter    

15 

83 

3 

11.  Indigestible  material  in  food.  When  we  say  that  "a  food 
is  digestible  we  generally  mean  that  when  taken  into  the 
alimentary  canal,  if  not  already  in  solution,  it  is  chemically 
acted  upon  by  the  digestive  juices  so  as  to  be  dissolved  and 
made  capable  of  being  absorbed  into  the  blood.  The  greater 
part  of  the  food  we  eat  consists  in  this  sense  of  digestible  sub- 
stances, but  many  foods  contain  a  certain  amount  of  indigest- 
ible material,  and  some  contain  a  very  considerable  amount. 

The  most  conspicuous  example  of  such  material  is  cellu- 
lose, a  member  of  the  same  group  of  carbohydrates  to  which 
starch  belongs.  It  occurs  in  almost  all  vegetable  foods ;  and 
since,  in  the  human  alimentary  canal,  cellulose  is  for  the 
most  part  unaffected,  it  cannot  be  absorbed  and  necessarily 


98 


THE  HUMAN  MECHANISM 


forms  an  important  part  of  the  feces.  Other  indigestible 
substances  are  the  outer  skin  of  animals  (for  example,  the 
skin  of  fowls),  and  certain  portions  of  the  connective  tissue 
of  meat. 

12.  Animal  and  vegetable  foods.  The  classification  of  foods 
into  animal  and  vegetable  not  only  describes  the  origin  of 
foods  from  the  two  great  kingdoms  of  living  things,  but 
also  defines  important  differences  between  them  with  refer- 
ence to  digestion.  These  differences  may  be  summed  up  as 
follows :  Animal  foods  are  generally  rich  in  proteins  and  poor 


FIG.  51.  Part  of  the  seed  of  the  bean 

Showing  the  larger  starch  granules  and 

the    finer    protein     granules    inclosed 

within  the  cellulose  cell  walls 


FIG.  52.   Section  of  potato 

Showing  starch  granules  inclosed 
within  the  cellulose  cell  walls 


in  carbohydrates,  while  vegetable  foods  are  generally  poor  in 
proteins  and  very  rich  in  carbohydrates,  especially  starch. 
In  the  second  place,  animal  foods  contain  relatively  little 
indigestible  material,  while  vegetable  foods,  as  they  occur 
in  nature,  contain  large  amounts  of  indigestible  cellulose. 
In  the  third  place,  the  digestible  materials  of  vegetable 
foods  (the  proteins,  carbohydrates,  and  fats)  are  often  con- 
tained within  a  plant  cell  which  is  surrounded  by  a  cellu- 
lose membrane  impermeable  to  the  digestive  juices;  before 
they  can  be  digested  this  membrane  must  be  ruptured  in 
one  way  or  another.  In  the  case  of  many  animal  foods,  on 


ALIMENTATION  AND  DIGESTION  99 

the  other  hand,  especially  meat  and  fat,  the  cells  (muscle 
fibers  and  fat  cells)  which  contain  the  essential  nutrients 
are  held  together  by  connective  tissue  made  up  largely  of 
fibers  of  an  albuminoid  nature.  These  fibers  are  soluble  in 
the  juices  of  the  stomach,  in  which  the  cellulose  which 
holds  together  the  vegetable  foods  is  insoluble.  The  full 
importance  of  these  differences  will  be  evident  before  we 
have  finished  the  study  of  digestion. 

13.  The  process  of  alimentation.  Before  corn,  wheat,  meat, 
vegetables,  and  other  food  materials  can  be  taken  into  the 
body  and  made  to  yield  up  to  it  the  material  and  power 
which  they  contain,  they  must,  in  most  cases,  undergo 
various  preparatory  or  preliminary  processes  or  treatments 
which  shall  make  them  easier  or  better  to  eat  or  more 
attractive.  The  most  familiar  of  these  processes  is  cooking, 
but  it  is  by  no  means  the  only  one.  In  the  case  of  animal 
food  the  animal  must  be  captured,  if  wild,  or  raised,  if 
domesticated.  It  must  be  killed,  skinned,  dressed,  cut  up, 
and  the  meat  in  many  cases  "  ripened "  by  keeping,  or 
"  cured "  by  smoking,  salting,  drying,  or  corning.  So,  also, 
with  plant  food,  such  as  cereals,  vegetables,  fruits,  nuts, 
and  the  like ;  these  must  first  be  found,  if  wild,  or  grown, 
if  domesticated.  They  must  then  be  separated  from  the 
rest  of  the  plant  —  threshed,  if  wheat,  rye,  oats,  or  barley; 
husked  and  shelled,  if  corn ;  dug  up  or  removed  from  the 
earth,  if  vegetables  like  potatoes,  celery,  radishes,  or  lettuce. 
Fruits  and  nuts  must  be  separated  or  picked  from  vine  or 
tree ;  milk  must  be  drawn  from  animals  ;  and  even  salt,  water, 
and  condiments  like  mustard  and  pepper  must  be  separated 
from  the  earth  or  the  sea  or  from  plants.  After  collection 
and  further  preparation  by  winnowing,  grinding,  or  cleaning, 
elaborate  cooking  is  applied  to  many  forms  of  food  before  it 
is  put  upon  the  table ;  and  even  then,  at  the  last  moment 
before  it  is  eaten,  a  further  separation,  as  of  meat  from  bone, 
must  be  made  either  by  the  carver  or  by  the  eater  himself. 


100  THE  HUMAN  MECHANISM 

To  this  entire  process  of  the  supply  and  preparation  of 
food  for  eating,  the  term  "alimentation"  may  be  conveniently 
applied.  Reflection  will  show  that  it  is  largely  a  process  of 
food  refining,  the  principal  result  being  a  concentration  of  the 
nutrients  at  every  step.  It  is  also  a  separation  of  the  com- 
paratively useful  from  the  comparatively  worthless  (as  food) ; 
and  just  here,  and  in  these  points,  —  concentration  and  the 
separation  of  good  from  poor  materials,  —  we  may  recognize  a 
true  process  of  digestion,  but  one  external  rather  than  in- 
ternal :  a  refining  in  the  field,  the  mill,  and  the  kitchen 
rather  than  in  the  stomach;  in  the  environment  rather  than 
within  the  organism. 

14.  The  ends  accomplished  by  digestion.  The  processes  of 
digestion  accomplish  three  chief  results:  First,  they  separate 
the  nutritious  and  therefore  important  part  of  the  food  from 
the  innutritious  and  therefore  useless.  This  process,  so  con- 
spicuous in  the  case  of  external  digestion,  is  continued  within 
the  alimentary  canal.  Second,  digestion  brings  the  solid  part 
of  the  food  into  solution  by  changing  insoluble  into  soluble 
substances.  This  is  necessary,  since  food  is  received  into  the 
body  proper  (that  is,  into  the  blood)  through  the  lining 
membranes  of  the  alimentary  canal,  and  in  order  that  it  may 
pass  through  these  membranes  it  must  be  dissolved.  In  the 
third  place,  digestion  transforms  the  food  as  eaten  into  com- 
pounds which  can  be  used  by  the  cells  of  the  body.  Common 
cane  sugar,  for  example,  is  very  soluble  and  can  be  absorbed 
into  the  blood,  but  the  cells  of  the  body  cannot  use  it.  In 
the  intestine  it  is  split  into  grape  sugar  and  fruit  sugar,  both 
of  which  can  be  used.  Similarly,  the  white  of  egg  (a  protein), 
though  soluble,  would  be  of  little,  if  any,  use  if  injected 
unchanged  into  the  blood;  in  the  alimentary  canal  it  is 
transformed  into  available  compounds.  It  will  be  helpful  to 
acquire  at  this  time  a  general  idea  of  the  chemical  structure 
of  two  of  our  most  important  foods  and  of  the  chemical 
changes  which  they  undergo  in  the  alimentary  canal. 


101 


15.  The  chemical  structure  of  the  starch  and  protein  mole- 
cules ;  cleavage  changes  during  digestion.  The  huge  molecules 
of  starch  and  protein  are  believed  by  chemists  to  consist  of 
a  large  number  of  much  smaller  molecules  linked  together 


FIG.  53.   Diagram  of  the  structure  of  molecules  of  starch  and  protein 

Starch  is  represented  as  formed  by  the  chemical  linking  together  of  many  like 
molecules  of  dextrose;  protein,  by  the  linking  together  of  many  unlike  mole- 
cules of  amino-acids.  Some  of  these  chemical  links  (indicated  by  the  arrows) 
are  broken  by  cleavage  more  easily  than  others.  Hence  cleavage  first  forms 
smaller  molecules  of  dextrines  from  starch  and  of  polypeptids  from  proteins. 
Ultimately  each  may  be  broken  up  into  its  constituent  molecules  of  dextrose  or 
amino-acids  respectively 

in  chemical  combination  (see  Fig.  53).  By  boiling  in  water 
containing  acid,  these  large  molecules  undergo  a  very  simple 
cleavage  into  their  component  molecules.  Starch  treated  in 
this  manner  yields  only  one  substance,  namely  dextrose 
(glucose,  or  grape  sugar  (C6H12O6)).  Protein,  on  the  other 
hand,  yields  a  much  greater  variety  of  compounds,  some 


102  THE  ti  u  MAN  MECHANISM 

twenty  or  more  in  number,  which,  though  differing  greatly 
from  one  another  in  most  respects,  have  in  common  one 
point  of  structure  in  virtue  of  which  they  are  known  as 
amino-acids.  In  the  chemical  laboratory  amino-acids  are 
readily  bound  together  to  form  peptids,  and  we  speak  of 
dipeptids,  tripeptids,  tetrapeptids,  and  polypeptids  accord- 
ing as  two,  three,  four,  or  many  amino-acids  enter  into  their 
formation.  It  is  now  thought  that  protein,  as  it  occurs  in 
nature,  is  essentially  a^very  complex  polypeptid. 

In  the  body  the  enzymes  of  the  digestive  juices  produce 
virtually  the  same  cleavage  in  starch  and  protein  as  that 
caused  by  boiling  with  acids,  and  the  chemical  action  upon 
the  food  within  the  stomach  and  intestine  consists  essentially 
in  breaking  up  the  starch  and  protein  into  their  component 
molecules  —  dextrose  in  the  one  case,  amino-acids  or  small 
peptids  in  the  other.  We  accordingly  find  that  as  the  result 
of  digestion  the  starch  we  eat  supplies  the  blood  (and  so  the 
body  cells)  with  only  one  substance,  namely  dextrose  (grape 
sugar),  and  the  value  of  starch  in  nutrition  is  limited  to  the 
nutritional  value  of  this  single  substance,  dextrose,  of  which 
it  is  composed.  Protein,  on  the  other  hand,  yields  twenty 
or  more  different  chemical  compounds,  each  with  its  own 
possibilities  of  chemical  action  in  the  cell.  Moreover,  indi- 
vidual proteins  differ  in  their  constituent  amino-acids;  a  given 
protein  may  be  entirely  lacking  in  one  or  more  amino-acids, 
or  it  may  have  one  or  more  present  in  very  small  or  very 
large  proportions.  The  nutritional  value  of  the  protein  is  con- 
sequently determined  by  the  possibilities  of  chemical  action 
of  its  constituent  amino-acids  and  by  the  quantity  of  each 
amino-acid  yielded  by  the  digestive  cleavage.  From  this  we 
can  readily  understand  why  protein  food  meets  a  wider  variety 
of  nutritional  requirements  than  does  starch  or  fat,  which  also 
yields  only  a  few  cleavage  products  upon  digestion. 

16.  Digestion  a  chain  of  events.  Before  entering  upon  the 
study  of  the  details  of  digestion  in  the  different  parts  of  the 


ALIMENTATION  AND  DIGESTION  103 

alimentary  canal,  a  suggestion  as  to  the  proper  point  of  view 
will  be  helpful.  While  it  is  true  that  each  part  of  the  diges- 
tive system  performs  functions  of  its  own,  it  is  also  true  that 
what  takes  place  in  one  part  is  dependent  on  what  takes 
place  in  others ;  digestion  in  the  mouth  has  reference  largely 
to  subsequent  work  in  the  stomach;  gastric  digestion,  in 
turn,  carries  one  step  further  the  refinement  of  the  food, 
which  it  thereby  prepares  for  what  is  to  take  place  in  the 
small  intestine ;  finally,  the  digestive  processes  of  the  large 
intestine  are  carried  out  normally  only  when  preceded  by  the 
proper  completion  of  those  of  the  small  intestine.  Digestion 
is  a  chain  of  events,  each  one  depending  upon  those  which  have 
gone  before  and,  to  a  large  extent,  upon  others  which  are  tak- 
ing place  at  the  same  time.  The  student  is  urged  to  keep 
this  in  view  in  the  study  of  all  the  digestive  processes. 

B.  DIGESTION  IN  THE  MOUTH.   ENZYMES 

17.  Stimulation  of  the  sense  of  taste  a  reflex  excitant  of 
the  flow  of  gastric  juice.  Digestion  in  the  mouth  prepares  for 
digestion  in  the  stomach,  in  the  first  place,  by  stimulating 
the  sense  of  taste  through  the  flavor  of  the  food,  for  the 
afferent  impulses  thus  aroused  play  a  very  important  r61e  in 
evoking  the  secretion  of  gastric  juice.  This  point  will  be 
more  fully  discussed  in  our  studies  of  gastric  digestion.  It 
is  referred  to  here  that  the  student  may  understand  that  far 
more  is  to  be  accomplished  by  the  stay  of  food  in  the  mouth 
than  its  mastication  and  mixture  with  saliva  preparatory  to 
the  act  of  swallowing.  We  might  imagine  a  meal  composed 
of  food  already  well  moistened  and  requiring  no  chewing, 
so  that  it  could  be  swallowed  immediately.  Such  a  meal 
might  have  all  the  nutrients  in  the  proper  proportions,  and 
yet,  from  the  very  fact  that  it  stays  so  short  a  time  in  the 
mouth,  it  may  not  sufficiently  arouse  sensations  of  taste  to 
evoke  an  adequate  reflex  secretion  of  gastric  juice.  It  is 


104  THE  HUMAN  MECHANISM 

perhaps  here  that  we  have  the  strongest  argument  against 
hasty  eating. 

18.  Mastication.     Digestion    in    the    mouth    prepares    for 
digestion  in  the  stomach,  in  the  second  place,  by  the  com- 
minution, or  grinding  down,  of  the  food  in  the  act  of  chew- 
ing.   When  this  is  properly  done  the  larger  food  masses  are 
broken  up  into  smaller  ones,  so  that  the  whole  is  made  more 
readily  accessible  to  the  subsequent  action  of  digestive  secre- 
tions.   The  small  intestine  has  almost  no  means  of  accom- 
plishing this  subdivision  of  the  food ;  the  stomach  can  do  it 
for  some  foods  easily,  for  others  with  difficulty,  while  against 
others  it  is  virtually  powerless.    Only  in  the  mouth  can  all 
foods   be  thoroughly   comminuted.    For   this  purpose   it   is 
necessary  to  keep  the  teeth  sound.1 

19.  Chemical   action  of   saliva.    Digestion   in   the  mouth 
presents  a  feature  which  is  characteristic  of  all  the  digestive 
processes ;  namely,  a  combination  of  the  mechanical  action  of 
some   form   of   muscular   movement   with   the   physical   and 
chemical  action  of  some  digestive  juice.    The  muscular  act 
of  chewing  and  the  secretion  of  saliva,  which  moistens  and 
acts  chemically  upon  the  food,  cooperate  to  reduce  the  food 
to  smaller  particles  and  to  change  part  of  it  into  other  sub- 
stances.   Neither  mastication  nor  insalivation,  acting  alone, 
would   be    as   effective   as   are  both  when   acting   together. 
We  shall  see  the  same  thing  more  strikingly  illustrated  in 
our  studies  of  gastric  and  intestinal  digestion. 

The  chemical  action  of  saliva  is  much  less  important  than 
that  of  other  digestive  juices,  but  it  is  typical  of  the  charac- 
ter of  all  of  them,  so  that  it  is  profitable  to  consider  it  at 
some  length.  Upon  proteins  and  fats  saliva  has  no  action 
whatever,  but  upon  starch  it  exerts  a  striking  and  readily 
demonstrable  influence.  To  demonstrate  the  effect  in  ques- 
tion some  starch  paste  should  be  prepared.  This  is  not  a 

1  The  structure  and  care  of  the  teeth  will  be  described  in  Part  II, 
Chap.  XXIII. 


ALIMENTATION  AND  DIGESTION  105 

clear  solution,  like  salt  or  sugar,  but  an  opalescent  liquid, 
which  does  not  become  clear  by  passing  through  ordinary 
filter  paper.  A  characteristic  test  for  starch  —  the  blue  color 
produced  when  a  few  drops  of  a  solution  of  iodine *  are 
added  to  it  —  may  be  used  to  detect  its  presence  in  the 
following  experiments : 

EXPERIMENT  I 

Two  test  tubes  or  small  beakers  containing  starch  paste  are  prepared. 
Collect  some  saliva  and  boil  half  of  it.  To  one  portion  of  the  starch 
paste  add  the  boiled  saliva  (after  it  has  again  cooled  to  the  room  tem- 
perature) ;  to  the  other  add  the  unboiled  saliva.  Mere  observation  will 
show  that  while  the  first  test  tube  remains  opalescent,  the  second  soon 
becomes  clear.  A  few  minutes  after  this  change  has  occurred,  a  little 
of  the  second  starch-saliva  mixture  may  be  removed,  diluted  with  water, 
and  tested  with  iodine ;  the  color  produced  is  no  longer  pure  blue,  but 
purplish ;  that  is,  a  mixture  of  red  and  blue.  Some  minutes  later  the 
iodine  test  gives  a  port-wine  red  color,  and  stiil  later  no  color  at  all. 
This  change  of  reaction  is  due  to  the  fact  that  the  saliva  has  changed 
the  starch  into  dextrine,  which  gives  the  red  color,  and  then  has  changed 
the  dextrine  into  a  substance  which  gives  no  color  with  iodine.2  Mean- 
while the  starch  in  the  first  test  tube  shows  no  change  either  in  its 
opalescent  appearance  or  in  its  original  blue  reaction  with  iodine. 

Boiling  the  saliva  has  destroyed  its  power  of  acting  on 
starch,  and  it  is  known  that  this  is  due  to  the  fact  that 
the  heat  has  destroyed  the  enzyme,  known  as  ptyalin,  or 
salivary  diastase,  which  has  the  power  of  changing  starch 
to  sugar. 

1  Made  by  dissolving  a  few  flakes  of  iodine  in  alcohol  or  in  an  aqueous 
solution  of  potassium  iodide. 

2  The  cleavage  of  the  starch  molecule  does  not  take  place  by  splitting  off 
successive  molecules  of  dextrose,  but  by  splitting  into  two  molecules,  each, 
let  us  say,  approximately  half  as  large  as  the  original  molecule.    By  some 
such  process  first  one,  then  another,  dextrine  successively  appears.    Con- 
tinuation of   the   cleavage   ultimately  gives   a  substance,   maltose,  which 
consists  of  two  molecules  of  dextrose  bound  together.   Finally,  the  maltose 
is  split  into  two  molecules  of  grape  sugar.    We  speak  of  the  dextrines  and 
maltose  as  intermediate  products,  and  of  the  dextrose  as  the  end  product,  of 
the  cleavage. 


106  THE  HUMAN  MECHANISM 

EXPERIMENT  II 

Let  us  now  inquire  what  has  become  of  the  starch  in  the  second  test 
tube.  The  solution  is  clear  and  has  a  sweetish  taste.  Moreover,  if  boiled 
with  a  mixture  of  sodium  hydroxide  and  a  few  drops  of  copper  sulphate, 
it  gives  a  red  precipitate,  indicating  the  presence  of  sugar.  These  sim- 
ple tests  then  prove  that  saliva  first  changes  starch  into  dextrine  and 
subsequently  changes  dextrine  into  sugar. 

EXPERIMENT  III 

Dilute  some  starch  paste  with  an  equal  volume  of  0.4  per  cent 
hydrochloric  acid  (which  will,  of  course,  make  a  0.2  per  cent  solution 
of  the  acid).  Now  add  a  few  drops  of  saliva.  It  will  be  found  that  no 
reaction  takes  place.  Saliva  will  not  act  in  an  acid  medium  of  this 
strength,  and  it  can  be  easily  shown  that  it  acts  most  vigorously  in  a 
neutral  or  faintly  alkaline  medium.  This  result  is  of  much  practical 
importance,  because  the  gastric  juice  contains  approximately  0.2  per 
cent  of  hydrochloric  acid  and  may  therefore  be  expected  to  interfere 
with  salivary  digestion. 

EXPERIMENT  IV 

Prepare  five  or  more  small  beakers  of  starch  paste  and  add  (best 
with  a  medicine  dropper)  to  the  first  a  drop  of  filtered  saliva,  to  the  sec- 
ond two  drops,  to  the  third  three  drops,  and  so  on;  then  observe  the 
time  required  in  each  case  for  the  disappearance  of  the  opalescence 
and  also  of  the  iodine  reaction.  This  experiment  will  show  that  while 
a  very  small  amount  of  saliva  will  transform  an  indefinite  amount  of 
starch  into  sugar,  the  more  saliva  there  is  present  the  more  rapidly  will 
the  transformation  occur ;  and  the  same  thing  is  true  of  all  enzymes. 
If  the  result  is  not  perfectly  clear  with  the  undiluted  saliva,  repeat, 
but  use  saliva  diluted  two  or  three  times  with  water. 

While  we  are  eating,  the  food  obviously  stays  too  short 
a  time  in  the  mouth  to  allow  the  conversion  of  any  large 
amount  of  its  starch  into  sugar  before  it  is  swallowed. 
Whatever  actual  work  the  saliva  may  do  in  bringing  about 
this  chemical  change  must  evidently  be  done  chiefly  in  the 
stomach,  and  this  will  be  studied  in  the  next  section. 

We  have  dwelt  at  length  upon  the  enzyme  action  of  saliva 
not  merely  for  its  own  sake  but  rather  because  the  behavior 


ALIMENTATION  AND  DIGESTION  107 

of  the  salivary  juice  is  typical  of  the  action  of  other  of  the 
digestive  juices  and  of  enzyme  action  in  general.  All  the 
other  juices  of  the  alimentary  canal,  with  the  single  excep- 
tion of  the  bile,  contain  enzymes,  and  it  will  greatly  help 
our  understanding  of  the  digestive  action  of  these  enzymes 
if  that  of  the  salivary  enzyme  be  first  mastered. 

Digestion  in  the  mouth,  then,  consists  first,  of  a  mechanical 
process  of  chewing,  by  which  food  is  crushed  or  comminuted; 
second,  of  a  physical  process  of  moistening,  by  which  dry 
foods  are  prepared  for  the  act  of  swallowing;  and  third,  of 
a  chemical  process,  the  chief  part  of  which  is  the  conversion 
of  starch  into  sugar  by  enzyme  action.  In  addition  to  this 
the  stimulation  of  the  sense  of  taste  reflexly  starts  the 
secretion  of  the  gastric  juice,  which  now  becomes  the  main 
chemical  agent  in  carrying  on  the  work  of  digestion.  To 
the  consideration  of  the  digestive  processes  in  the  stomach 
we  may  now  devote  our  attention. 

C.  DIGESTION  IN  THE  STOMACH 

According  to  popular  ideas  the  stomach  is  the  chief  organ 
of  digestion;  in  fact,  however,  it  is  an  organ  in  which  the 
food  which  has  been  swallowed  is  temporarily  stored  while 
undergoing  a  preliminary  preparation  for  the  more  impor- 
tant changes  which  are  to  take  place  in  the  intestine.  In  this 
preparatory  process,  to  be  sure,  some  of  the  food  is  inciden- 
tally changed  into  those  forms  in  which  it  passes  into  the 
blood,  but  this  action  is  incidental  and  subordinate  to  the 
main  function. 

20.  Form  and  structure  of  the  stomach.  The  stomach  is  a 
large  pouch  into  which  open  two  tubes  —  the  oesophagus 
(gullet)  toward  the  left  side  and  the  intestine  on  the  right 
(see  Fig.  54).  The  two  regions  into  which  these  tubes  open 
are  different  in  structure  and  are  known  as  the  cardiac  (left) 
and  pyloric  (right)  portions  of  the  stomach;  the  cardiac 


108 


THE  HUMAN  MECHANISM 


portion  differs  from  the  pyloric  portion  in  having  greater  di- 
ameter and  thinner  walls.  The  entire  inner  surface  is  lined  by 
the  mucous  membrane  some  three  or  more  millimeters  in  thick- 
ness, crowded  with  comparatively  simple  glands  which  pour 
their  secretion,  the  gastric  juice,  into  the  stomach  very  much  as 
sweat  glands  discharge  perspiration  on  the  skin  (see  Fig.  55). 

CEsophagus IT- 

Cardiac     \ 
Muscle 
Gall_ 
Bladder 


Bile  Duct 


Intestine 


From  the  Liver 
\       \    Pylorus 


'Pancreatic  Duct 


FIG.  54.    Stomach,  beginning  of  small  intestine,  and  entrance  of  bile  and 
pancreatic  ducts 

During  digestion  the  bile  flows  directly  from  the  liver  into  the  intestine ;  at  other 

times  the  opening  of  the  bile  duct  is  closed  and  the  bile  passes  into  the  gall 

bladder,  where  it  is  stored 

The  glandular  membrane  is  one  of  the  two  principal 
components  of  the  stomach  wall ;  the  other  is  the  muscular 
or  contractile  tissue,  which  forms  a  second  coat  outside  the 
other,  arid  closely  united  to  it  by  connective  tissue  con- 
taining the  larger  blood  vessels,  lymphatics,  nerves,  etc.1 
The  muscular  coat  is  comparatively  thin  in  the  cardiac  region 


1  Fig.  63  (large  intestine)  shows  in  cross  section  somewhat  the  same 
arrangement  of  mucous  and  muscular  coats  as  in  the  wall  of  the  stomach. 


ALIMENTATION  AND  DIGESTION 


109 


and  comparatively  thick  in  the  pyloric,  the  thickening  in  the 
latter  region  being  caused  chiefly  by  muscle  fibers  circularly 
arranged. 

21.  The  gastric  juice.  The  gastric  juice  is  a  clear,  thin, 
colorless  liquid  which  contains,  among  other  things,  about 
0.2-0.3  per  cent  of  hydrochloric 
acid  and  certain  enzymes.  Upon 
starch  it  has  no  action  whatever, 
nor  has  it  any  action  on  fats,  unless 
the  fat  is  in  the  form  of  an  emulsion 
(that  is,  very  fine  drops  of  oil  sus- 
pended in  water,  as  in  milk  or  may- 
onnaise dressing)  ;  indeed,  the  very 
limited  power  of  gastric  juice  to  at- 
tack fat  is  a  matter  of  considerable 
importance  in  dietetics.  Its  main 
chemical  action  is  upon  the  proteins, 
which  under  its  influence  undergo 
cleavage  into  proteases  and  peptones. 
The  proteoses  and  peptones,  like 

the  original  protein,  are  polypeptids  (p.  102),  but  of  smaller 
molecular  size.  They  are  not  coagulated  by  heat,  and  most 
of  them  are  soluble. 

EXPERIMENTS 

Prepare  some  artificial  gastric  juice  as  follows  :  To  one  quart  of  water 
add  7  or  8  cc.  of  concentrated  hydrochloric  acid  and  to  this  a  little  active 
pepsin,  which  may  be  obtained  at  any  drug  store.  Pepsin  is  extracted 
from  the  stomach  and  is  the  most  important  of  its  enzymes.  A  solution 
of  pepsin  in  the  given  strength  of  hydrochloric  acid  is  virtually  gastric 
juice.  Try  the  effect  of  this  on  the  following  substances  by  placing  each 
in  a  half  tumblerful  of  the  juice.  To  get  the  complete  effect  the  mixture 
should  be  set  aside  for  twenty-four  hours  and  tests  made  the  next  day. 
Observations  should  be  made  during  the  first  hour  or  two.  If  the  digest- 
ing mixture  can  be  kept  in  a  warm  place  (90°-100°  F.),  the  action  will 
be  more  rapid  and  the  results  more  satisfactory.  The  digestions  can 
best  be  carried  out  in  corked  4-ounce  bottles,  which  should  be  shaken 


FIG.  55.  The  inner  surface  of 

the  stomach  (magnified  about 

20  diameters) 

Showing  the  openings  of  the 
glands.  The  lining  glandular 
membrane  is  thrown  into  folds 


110  THE  HUMAN  MECHANISM 

occasionally  to  secure  better  contact  of  the  digestive  juice  with  the 
material  undergoing  digestion. 

1.  The  white  of  soft-boiled  (3-4  minutes)  egg.    This  is  composed 
mostly  of  protein ;    it  will  be  dissolved.    Into  what  is  the  egg  white 
changed  ? 

2.  A  piece   of  tendon,  which  can  be  obtained  from  any  butcher. 
This  is  composed  of  the  kind  of  fibers  which  are  found  in  the  con- 
nective tissues  holding  the  cells  together  (see  Chap.  III).    The  tendon 
first  swells,  then  gradually  disintegrates,  its  protein  (albuminoid,  p.  94) 
fibers  going  into  solution.    A  small  residue  will  be  left. 

3.  A  piece  of  the  lean  of  rare  meat  cut  or  chopped  into  small  pieces. 
The  meat  will  disintegrate,  owing  to  the  solution  of  its  connective  tissue 
fibers ;  then  the  protein  muscle  fibers  will  go  into  solution,  being  changed 
into  soluble  peptids. 

4.  A  piece  of  lean  of  well-cooked  meat.    The  result  will  be  much  like 
that  in  (3)  except  that  it  will  probably  take  longer  to  bring  the  muscle 
fibers  into  solution. 

5.  Some  jelly   (made  from  gelatin)  which  has  set.    This  will  be 
gradually  dissolved. 

6.  Some  fat  (not  gristle)  of  beef.    The  mass  will  disintegrate  for  the 
same  reason  as  in  the  case  of  meat.    The  fat  itself  will  be  unacted  on, 
but  will  rise  to  the  top,  where  it  may  form  a  layer  of  fat  or  oil. 

7.  A  piece  of  bread.    This  consists  of  starch,  fat,  etc.  held  togethei 
by  the  tenacious  gluten  (a  protein).    As  the  gluten  is  dissolved  by  the 
gastric  juice  the  undissolved  starch,  fat,  etc.  is  set  free. 

8.  Some  starch  paste.    No  action. 

9.  Some  fried  steak.  Note  the  prolongation  of  the  period  of  digestion. 

Instructive  experiments  may  also  be  made  with  cheese, 
sweetbreads,  potatoes,  peas,  etc.  They  would  all  bring  out 
the  main  points  in  the  action  of  the  gastric  juice.  These 
may  be  summed  up  as  follows:  Gastric  juice  has  no  effect 
upon  pure  fats  (although  it  plays  an  important  part  in  the 
digestion  of  adipose  tissue1),  nor  upon  carbohydrates,  such 
as  starch  or  sugar.  Its  part  in  digestion  consists  in  its  action 

1  The  fat  of  meat  consists  of  connective  tissue  whose  cells  are  greatly 
swollen  with  drops  of  fat.  In  typical  adipose  tissue  the  connective-tissue 
cell  becomes  one  large  fat  droplet  surrounded  by  the  thin  layer  of  the  cell 
cytoplasm  with  its  nucleus.  These  fffat  cells,"  like  the  muscle  fibers  of 
meat,  are  thus  held  together  by  the  fibers  of  connective  tissue  and  are  set 
free  when  the  latter  are  digested  and  dissolved  away  by  the  gastric  juice 
(see  Figs.  90-92). 


ALIMENTATION  AND  DIGESTION  111 

upon  the  proteins  of  the  food  and  especially  upon  those 
proteins  (albuminoids)  which  make  up  the  connective  tissue 
of  animal  foods.  By  dissolving  this  connective  tissue,  which 
holds  together  the  muscle  fibers,  fat  cells,  etc.,  animal  food 
is  considerably  subdivided  and  made  to  present  a  greatly  in- 
creased surface  to  the  further  action  of  digestive  juices.  It 
is  also  well  to  remember  that  the  gastric  juice  dissolves  con- 
nective tissue  much  more  rapidly  than  does,  any  other  of  the 
digestive  juices  and  that  this  action  upon  connective  tissue 
is  really  more  important  than  that  upon  other  proteins,  al- 
though the  latter  is  usually  more  emphasized.  Other  proteins 
not  acted  on  in  the  stomach  are  rapidly  digested  by  the 
pancreatic  juice  in  the  intestine;  connective  tissue,  on  the 
contrary,  escaping  solution  in  the  stomach,  is  dissolved  but 
.slowly  in  the  intestine. 

The  student  is,  however,  warned  against  supposing  that 
because  gastric  juice  is  able  to  transform  the  proteins  of  the 
food  to  peptids,  it  actually  does  exert  this  action  upon  all 
the  protein  eaten.  In  point  of  fact,  as  protein  foods  are 
divided  into  smaller  and  smaller  particles  in  the  stomach, 
they  are  discharged  into  the  intestine,  where  their  digestion 
is  completed  by  the  pancreatic  juice.  In  man  the  pancreatic, 
and  not  the  gastric,  juice  is  the  main  agent  of  protein  digestion. 

22.  The  stomach  at  work.  Having  now  gained  a  general 
idea  of  the  chemical  changes  which  occur  in  the  stomach,  we 
may  proceed  to  consider  what  actually  happens  when  food 
enters  that  organ.  And  here  our  knowledge  has  been  gained 
partly  by  examining  the  gastric  contents  at  different  periods 
of  digestion,  partly  by  observing  the  movements  of  the 
stomach  by  the  aid  of  the  Rbntgen  rays,  and  partly  by 
other  means. 

As  soon  as  food  enters  the  stomach,  and  even  while  it  is 
still  in  the  mouth,  the  gastric  glands  begin  to  discharge  the 
gastric  juice,  and  continue  to  do  so  during  the  four  or  more 
hours  of  gastric  digestion.  When  the  meal  is  fluid  or  is  small 


112 


THE  HUMAN  MECHANISM 


in  amount,  this  gastric  juice  is  thoroughly  mixed  with  it; 
when,  however,  the  food  is  more  or  less  solid  and  bulky,  only 
the  outer  layers,  which  are  in  immediate  contact  with  the 
walls  of  the  stomach,  are  mixed  with  the  juice.  At  least  this 

is  true  at  the  cardiac  end ;  the 
cavity  of  the  pyloric  portion  is  so 
small  and  the  amount  of  move- 
ment there  so  great  that  all  por- 
tions of  the  pyloric  contents  are 
thoroughly  mixed  with  gastric 
juice ;  in  the  much  larger  cardiac 
portion  the  central  mass  of  the 
food  may  receive  no  gastric  juice 
and  thus  remain,  for  an  hour  or 
more  after  the  meal,  neutral  or 
alkaline  in  reaction.  Under  these 
circumstances  very  considerable 
amounts  of  starch  may  continue 
to  undergo  the  salivary  digestion 
begun  in  the  mouth. 

Any  chemical  action  is  aided  by 
agitation,  since  the  reacting  com- 
pounds are  thus  brought  into  more 
intimate  union ;  and  observation 
of  the  working  stomach  shows 
that  while  the  cardiac  portion 
makes  no  movements,  but  merely 
keeps  up  a  steady  contraction 
and  thereby  exerts  a  moderate 
pressure  upon  its  contents,  the  pyloric  portion  executes,  from 
a  very  early  stage  of  digestion  and  throughout  the  whole  proc- 
ess, a  series  of  contractions  which  gradually  bring  about  a 
thorough  mixture  of  the  contents  and  rub  down  the  softened 
food  into  smaller  and  smaller  masses.  These  contractions 
consist  of  rings  of  constriction  which  arise  at  the  beginning 


FIG.  56.  Outline  of  the  contents 

of  the  stomach  of  a  cat  at  three 

stages  of  the  digestion  of  a  meal 

taken  about  11  A.M. 

Showing  the  peristaltic  constric- 
tions which  pass  over  the  pyloric 
portion  and  the  diminution  of  the 
quantity  of  food  in  the  cardiac 
end.  (Full  description  given  in 
sect.  22) 


ALIMENTATION  AND  DIGESTION  113 

of  the  pyloric  portion  and  pass  onward  to  the  pylorus  itself, 
a  new  ring  beginning  about  once  every  ten  seconds  and  con- 
suming from  thirty  to  forty  seconds  in  passing  to  the  pylorus. 
Consequently  there  are  always  two  or  more  slowly  moving 
rings  in  the  pyloric  end  of  the  stomach  at  one  time.1 

The  pyloric  end  of  the  stomach  is  thus  the  seat  of  a  combined 
chemical  and  mechanical  action  on  the  food.  The  vegetable 
foods  are  softened,  while  the  connective  tissue  of  the  animal 
foods  is  dissolved  away ;  in  addition,  the  food  is  mixed  with 
a  considerable  amount  of  liquid  supplied  by  the  secretion  of 
gastric  juice.  The  contents  of  the  pyloric  end  of  the  stomach 
thus  ultimately  come  to  consist  of  minute  solid  masses  sus- 
pended in  a  liquid,  the  consistency  of  the  whole  being  that 
of  moderately  thick  pea  soup.  This  product  of  the  work  of 
the  stomach  is  known  as  chyme. 

23.  The  expulsion  of  chyme  into  the  intestine.  The  open- 
ings of  the  oesophagus  and  intestine  into  the  stomach  are 
usually  closed ;  the  former  is  opened  normally  only  during 
the  act  of  swallowing,  while  the  latter  opens  at  irregular 
intervals  during  the  process  of  gastric  digestion.  The  open- 
ing of  the  pylorus  allows  the  rings  of  constriction  moving 
over  that  region  of  the  stomach  to  discharge  the  semifluid 
chyme  into  the  intestine.  If,  however,  a  large  mass  of  solid 
food  arrives  and  is  driven  against  the  walls,  the  pylorus 
reflexly  closes,  thus  guarding  the  entrance  of  the  intestine 
from  the  passage  of  food  not  yet  ready  for  intestinal  diges- 
tion. The  pressure  exerted  by  the  sustained  contraction  of 
the  walls  of  the  cardiac  end  of  the  stomach  adds  to  the 
food  in  the  pyloric  region  new  portions  from  time  to  time, 
and  the  same  combined  chemical  and  mechanical  process 
already  described  is  continued  until  the  whole  mass  is 
reduced  to  chyme  and  driven  into  the  intestine. 

1  These  movements  of  the  stomach  and  intestine  are  well  shown  in 
zoetrope  figures,  which  may  be  obtained  from  the  Harvard  Apparatus 
Company,  Back  Bay  Post  Office,  Boston. 


114  THE  HUMAN  MECHANISM 

This  brief  sketch  of  the  working  of  the  stomach  shows 
that  this  organ  serves  the  two  main  functions  of  storing  the 
food  and  of  making  it  more  accessible  to  the  digestive  fluids 
of  the  intestine.  When  the  chyme  is  delivered  to  the  intes- 
tine, the  mechanical  difficulties  in  the  way  of  absorption  are 
practically  gone ;  the  surface  of  the  food  exposed  to  diges- 
tive action  is  now  immensely  increased  by  its  subdivision, 
and  the  work  remaining  for  the  intestine  is  almost  wholly 
the  chemical  duty  of  changing  the  constituents  of  the  chyme 
into  substances  which  are  soluble  and  ready  for  absorption. 

Serious  troubles  arise  when,  for  one  reason  or  another, 
gastric  digestion  goes  wrong,  because  the  subsequent  proc- 
esses of  digestion  are  largely  dependent  upon  the  preparation 
which  the  food  receives  in  the  stomach.  Gastric  digestion 
may  be  impaired  in  one  of  three  ways :  first,  the  gastric 
juice  may  not  be  secreted  in  proper  amount  or  proper 
strength;  second,  the  stomach  may  not  execute  its  move- 
ments efficiently ;  third,  the  gastric  juice  secreted  may  not 
be  able  to  get  at  the  food  readily,  owing  to  improper  cook- 
ing or  insufficient  mastication.  The  study  of  the  conditions 
which  produce  these  troubles  —  which  taken  together  consti- 
tute one  form  of  indigestion,  or  dyspepsia — will  be  postponed 
to  the  chapter  on  the  Hygiene  of  Feeding  (Part  II). 

24.  The  stimulus  to  the  secretion  of  the  gastric  juice.  The 
first  requirement  for  the  work  of  the  stomach  is  the  secre- 
tion of  sufficient  gastric  juice.  Of  late  years  the  brilliant 
researches  of  physiologists  have  shown  that  the  secretion  of 
gastric  juice  is  called  forth  in  three  ways : 

1.  The  "psychic"  secretion.  When  agreeable  or  appetizing 
food  is  offered  to  an  animal,  and  especially  when  such  food 
is  taken  into  the  mouth,  a  secretion  of  gastric  juice  follows, 
which  may  continue  for  fifteen  minutes  or  more.  This  secre- 
tion occurs  when  the  food  has  been  in  the  mouth  only  ten 
or  fifteen  seconds  and  even  when  it  is  merely  offered  to  a 
hungry  animal  and  not  taken  into  the  mouth  at  all.  Again, 


ALIMENTATION  AND  DIGESTION 


115 


1     23456789    10 


it  occurs  only  when  the  animal  is  conscious;  for  if  food  be 
introduced  into  the  stomach  of  a  sleeping  dog,  it  evokes  only 
the  most  scanty  secretion  of  gastric  juice  after  the  animal 
has  awakened.  Moreover,  both  the  amount  and  the  efficiency 
of  the  juice  secreted  vary  directly  with  the  enjoyment  of  the 
meal.  When  meat  is  given  to  a  dog  which  is  not  hungry, 
no  such  abundant  secretion  of 
gastric  juice  occurs  as  during 
hunger. 

It  is  clear  that  we  have  here 
to  deal  with  a  nervous  process 
more  complicated  than  the  sim- 
ple reflex,  and  that  the  efferent 
discharge  to  the  stomach  occurs 
as  the  result  of  nervous  proc- 
esses taking  place  in  the  brain 
in  connection  with  the  enjoy- 
ment of  food.  In  other  words, 
the  more  the  food  is  desired 
or  enjoyed,  the  more  efficient 
will  be  this  secretion  of  the 
gastric  juice. 

It  is  known  that  this  "  psy- 
chic"   secretion  will   continue 
for  several  hours  after  an  ordi- 
nary meal,  increasing  in  amount  during  the  first  hour  or  more 
and  gradually  diminishing  from  that  time  onward  (Fig.  57). 

2.  Stimulation  of  the  stomach  by  constituents  of  certain  foods. 
We  have  seen  that  the  direct  introduction  of  food  into  the 
stomach  (for  example,  into  the  stomach  of  a  sleeping  animal) 
does  not  of  itself  evoke  a  secretion  of  gastric  juice.  Some 
foods,  however,  contain  substances  which  do  evoke  such  a 
secretion,  the  most  important  of  these  being  certain  con- 
stituents of  meat.  Bouillon,  for  example,  which  is  an  extract 
of  meat,  directly  excites  the  wall  of  the  stomach  to  secrete. 


FIG.  57.  The  curve  of  the  "psychic" 
secretion  of  gastric  juice 

Vertical  lines  represent  half-hour  pe- 
riods after  taking  the  meal ;  horizon- 
tal lines,  relative  amounts  of  gastric 
juice  secreted 


116  THE  HUMAN  MECHANISM 

This  is  a  reason  for  introducing  the  soup  early  at  a  course 
dinner.  Meat  extracts  and  meat  juices  are  the  most  effective 
food  constituents  for  this  purpose ;  milk  and  water  are  far 
less  effective,  while  most  foods,  notably  bread,  white  of  eggs, 
etc.,  have  no  such  effect  at  all. 

3.  Stimulation  of  the  stomach  itself  by  the  products  of  protein 
digestion.  Although  the  mere  contact  of  most  foods  with  the 
lining  of  the  stomach  does  not  evoke  a  secretion  of  gastric 
juice,  yet  it  is  known  that  after  digestion  has  been  begun 
by  the  action  of  the  "  psychic "  secretion,  certain  of  the 
products  of  protein  digestion  arouse  a  second  secretion  ,by 
acting  directly  on  the  lining  of  the  stomach.  This  second 
secretion  increases  in  amount  as  the  first  (or  "  psychic " ) 
secretion  diminishes,  and  continues  throughout  the  remaining 
period  of  gastric  digestion. 

To  sum  up :  The  secretion  of  the  gastric  juice  is  initiated 
by  a  complicated  series  of  nervous  processes  connected  with 
the  enjoyment  of  the  food  while  it  is  being  taken  and  masti- 
cated ;  this  is  aided  to  some  extent  by  direct  stimulation  of 
the  lining  of  the  stomach  by  a  few  food  constituents,  notably 
the  extractives  of  meat.  The  gastric  juice  thus  secreted  acts 
upon  the  proteins  of  the  food  and  produces  from  them  diges- 
tive products  which  directly  stimulate  the  stomach  to  secrete 
and,  in  fact,  maintain  the  secretion  to  the  end  of  the  period 
of  gastric  digestion.  Without  the  "psychic"  secretion  pro- 
teins are  not  digested  fast  enough  to  induce  sufficient  sub- 
sequent secretion ;  without  the  stimulus  of  the  products  of 
protein  digestion  the  "  psychic  "  secretion  does  not  suffice  to 
complete  the  digestion  of  a  hearty  meal  —  a  labor  which  may 
require  four  or  five  hours.1 

1  What  we  have  called  the  "psychic"  secretion  is  probably  an  uncondi- 
tioned reflex  from  the  mouth,  reenforced  by  a  conditioned  reflex  involving  the 
action  of  the  cerebrum  ;  the  stimulation  by  the  products  of  protein  digestion 
and  possibly  that  by  meat  extracts,  on  the  other  hand,  is  probably  due  to 
a  hormone  (p.  89)  liberated  in  the  mucous  membrane  of  the  pyloric  region, 
thence  passing  into  the  blood,  and  so  stimulating  the  gastric  glands  to  secrete. 


ALIMENTATION  AND  DIGESTION  117 

D.  DIGESTION  AND  ABSORPTION  IN  THE  SMALL  INTESTINE 

AND   IN   THE   LARGE   INTESTINE 

Every  few  minutes  during  the  process  of  gastric  digestion 
the  pylorus  opens  and  the  stomach  forces  a  few  cubic  centi- 
meters of  chyme  into  the  intestine.  Chyme,  which  consists 
of  water  holding  in  solution  certain  products  of  digestion, 
and  carrying  in  suspension  larger  quantities  of  undissolved 
matter,  has  the  consistency  of  moderately  thick  pea  soup. 
The  suspended  matter  consists,  among  other  things,  of  small 
bundles  of  muscle  fibers  (from  meat),  fat  melted  by  the 
heat  of  the  body  and  set  free  from  adipose  tissue  by  the 
digestion  of  its  connective  tissue,  bits  of  coagulated  protein, 
such  as  casein  from  milk  or  the  white  of  egg,  together  with 
starches,  fats,  and  proteins  of  animal  or  vegetable  foods. 
Thus  far  the  digestive  processes  in  the  mouth  and  stomach 
have  been  essentially  preparatory  to  the  main  chemical  work 
of  digestion,  which  takes  place  in  the  small  intestine.  The  finely 
subdivided  food  is  now  attacked  by  the  digestive  juices  of 
the  small  intestine  brought  into  solution,  and  otherwise  made 
ready  for  absorption  into  the  blood. 

25.  The  general  structure  of  the  intestine;  the  pancreas 
and  the  liver.  The  main  functions  of  the  intestine,  like  those 
of  the  stomach,  are  indicated  in  the  structure  of  two  of  its 
coats,  the  muscular  coat  and  the  glandular  mucous  mem- 
brane. The  fibers  of  the  former  are  arranged  in  two  layers  — 
an  inner  layer  in  which  they  are  circularly  disposed  around 
the  mucous  membrane  (see  Fig.  58),  and  a  much  thinner 
outer  layer  in  which  they  run  lengthwise.  The  contraction, 
or  shortening,  of  the  circular  fibers  constricts  the  bore,  or 
lumen,  of  the  tube,  and  this  constriction  of  the  intestinal 
tube  is  the  most  important  work  of  the  muscular  coat. 
Sometimes  the  constriction  is  confined  to  one  place ;  at  other 
times  it  moves  along  the  tube,  pushing  before  it  the  contents. 
(See  under  Peristalsis,  p.  125.) 


118  THE  HUMAN  MECHANISM 

In  the  structure  of  the  inner  or  mucous  membrane  two 
points  are  of  importance  to  us.  In  the  first  place,  numerous 
simple  tubular  glands  discharge  into  the  intestinal  tube  an 
important  digestive  juice,  the  intestinal  juice ;  in  the  second 
place,  fingerlike  processes,  or  villi  (0.5—0.7  mm.  long  by 
0.1  mm.  thick),  arise  from  its  surface  and  project  into  the 
intestinal  cavity.  These  are  important  organs  of  absorp- 
tion. The  entire  surface  of  the  villi,  the  glands,  and  the 
plane  surface  of  the  intestine  between  these  structures  is 
lined  with  a  continuous  membrane  composed  of  colum- 
nar cells,  which  separates  blood  vessels  and  lymphatics  •  in 
the  intestinal  wall  from  the  cavity  of  the  intestine  (see 
Fig.  59).  The  products  of  digestion  must  therefore  pass  either 
through  these  cells  or  between  them  to  enter  the  blood 
or  lymph. 

The  intestine  is  some  twenty  or  twenty-five  feet  in  length, 
and  the  intestinal  glands  (Fig.  58)  constantly  secrete  intes- 
tinal juice  upon  the  contents  as  they  are  slowly  moved  along 
the  tube.  Two  other  juices  are  added  to  the  intestinal  con- 
tents almost  immediately  after  then*  entrance  to  the  upper 
part  of  the  small  intestine.  These  are  the  pancreatic  juice 
and  the  bile,  which  are  secreted,  respectively,  by  the  pancreas 
and  the  liver.  The  entrance  of  the  ducts  of  these  glands  is 
shown  in  Fig.  54.  It  is  not  necessary  for  our  present  purpose 
to  describe  the  minute  structure  of  these  organs;  it  is  enough 
for  the  student  to  understand  that  they  are  glands  (p.  29) 
which  pour  their  secretions  through  ducts  into  the  intestine 
very  much  as  the  salivary  glands  pour  their  secretions  into 
the  mouth. 

26.  The  mechanism  of  secretion  of  pancreatic  juice,  bile, 
and  intestinal  juice.  The  mechanism  which  evokes  the  secre- 
tion of  the  pancreatic  juice  has  already  been  described 
(p.  89).  It  will  be  recalled  that  the  lining  cells  of  the 
intestine  immediately  beyond  the  pylorus  (duodenum)  con- 
tain a  material  which  when  acted  upon  by  the  hydrochloric 


ALIMENTATION  AND  DIGESTION 


119 


acid  of  the  chyme  is  transformed  into  the  hormone  secretin. 
This  is  absorbed  into  the  blood  and  chemically  excites  the 
pancreas  to  secrete. 

The  secretion  of  bile  by  the  liver  is  continuous,  although 
it  is  greater  at  one  time  than  at  another.  Circular  muscle 
fibers  at  the  mouth  of  the 
bile  duct  close  the  opening 
into  the  intestine  when  bile 
is  not  needed  there ;  at  such 
times  the  bile  secreted  accu- 
mulates in  the  gall  bladder. 
During  active  digestion  the 
mouth  of  the  bile  duct  re- 
mains open  and  the  bile  flows 
immediately  into  the  intestine. 

Little  is  known  of  the  fac- 
tors determining  the  secretion 
of  intestinal  juice,  but  it  prob- 
ably is  continuously  secreted, 
at  least  so  long  as  food  is  in 
the  intestine.  Thus  each  con- 
signment of  chyme  from  the 
stomach  receives  its  share  of 
pancreatic  juice  and  bile  soon 
after  it  enters  the  duodenum, 
and  then  subsequently  re- 
ceives continuous  additions  of 
intestinal  juice  as  it  is  passed 
along  the  intestinal  tube  by 
the  action  of  the  muscular 
coat  presently  to  be  described. 

27.  The  pancreatic  juice  is  a  strongly  alkaline  liquid  and 
consequently,  when  mixed  with  the  acid  chyme,  neutralizes 
most,  if  not  all,  of  the  hydrochloric  acid  of  the  chyme.  Thus  it 
happens  that  while  the  food  in  the  stomach  is  strongly  acid, 


FIG.  58.  Longitudinal  section  of  the 
small  intestine 

The  submucous  coat  consists  of  con- 
nective tissue  and  contains  the  larger 
blood  vessels  from  which  the  mucous 
and  muscular  coats  are  supplied  with 
blood 


120 


THE  HUMAN  MECHANISM 


A 


B 


in  the  intestine  it  becomes  at  once  more  nearly  neutral  or 
even  alkaline.  Since  pepsin  acts  only  in  an  acid  medium, 
the  gastric  juice  now  becomes  inactive  and  is  soon  de- 
stroyed by  the  pancreatic  juice,  so  that  it  plays  no  further 

r61e  in  protein  digestion.  This 
is  henceforward  carried  on  by 
an  enzyme  of  the  pancreatic 
juice,  trypsin,  which  acts  most 
vigorously  in  a  neutral  or 
slightly  alkaline  medium.  It 
forms  from  the  proteins  of  the 
food  the  same  general  class  of 
peptone-like  substances  pro- 
duced by  the  action  of  the 
gastric  juice,  but  carries  this 
cleavage  further  into  smaller 
peptids  and  even  to  some  ex- 
tent to  the  constituent  amino- 
acids.  Trypsin  continues  the 
digestion  of  proteins  begun  by 
FIG.  59.  Longitudinal  section  of  pepsin.  Indeed,  in  some  cases 
the  tip  of  a  villas  tjie  preliminary  action  of  pep- 

Showing  the  columnar  lining  cells     sin    •     necessary,  since  trypsin 

B   through   which  the  products  of  J'  J^ 

digestion  must  pass  on  their  way  to      does    not    act    SO    readily    Upon 

the  °"inal     «>tein  as  it  does 


columnar  cells  and  the  vessels  is  in-  upon     the     earlier    products    of 
dicated  diagrammatically  and  with-  t:p     rli  option  •     nnon     thpsp 

out  showing  its  structure.    A,  cell  P6Pt] 

which  manufactures  mucus  ;  C,  cap-  cleavage  products,  however,   its 

illaries;  D.  lacteal,  or  lymphatic  ^^    jg   mog(.   vigorous. 

In  addition  to  trypsin  the  pancreatic  juice  contains  at  least 
two  other  important  enzymes.  One  of  them,  amylopsin,  is 
practically  identical  with  the  ptyalin  of  the  saliva  and 
changes  starch  into  sugar  much  as  happens  in  salivary  diges- 
tion. The  other  enzyme,  lipase,  acts  upon  fats,  changing 
them  into  fatty  acids  and  glycerin.  We  cannot  go  into  the 


ALIMENTATION  AND  DIGESTION  121 

details  of  the  somewhat  complicated  digestion  of  fats.  The 
change,  like  that  of  proteins  into  peptones  and  of  starches 
into  sugar,  involves  the  formation  of  a  smaller  molecule, 
either  of  fatty  acids  or  soaps,  or  both,  and  it  is  probably 
in  these  forms  that  fats  are  received  from  the  intestine  by 
the  villi. 

The  pancreatic  juice  thus  contains  a  special  enzyme  for 
each  of  the  three  great  classes  of  nutrients — proteins,  fats, 
and  carbohydrates  —  and  thoroughly  completes  their  diges- 
tion after  they  have  undergone  the  preparatory  processes 
effected  by  cooking,  mastication,  and  gastric  digestion. 
Pancreatic  juice  is  by  far  the  most  important  of  the  digestive 
juices  in  producing  the  chemical  changes  of  digestion.  In  this 
respect,  also,  we  may  say  it  is  of  primary  importance  in  the 
work  of  intestinal  digestion,  the -other  two  juices,  the  bile 
and  the  intestinal  juice,  acting  as  aids  in  its  work. 

28.  The  bile  contains  no  enzymes  of  importance  in  diges- 
tion.   It  is   in   fact   partly   an  excretion,   some   of   its   con- 
stituents being  waste   products  which   are  poured   into  the 
intestine  only  to  be  ultimately  discharged  from  the  rectum. 
Other  constituents  of  the  bile  play  an  important  role  in  the 
digestion  and  absorption  of  fats,  as  is  shown  by  the  fact  that 
if  bile  be  prevented  from  entering  the  intestine,  from  forty 
to  sixty  per  cent  of  the  fat  eaten  fails  of  absorption  and  is 
discharged  with  the  feces.    It  is  probable  that  this  is  because 
certain  soaps  formed  in  pancreatic  digestion  are  not  soluble 
unless  bile  is  present.    When  these  soaps  are  not  dissolved, 
they  are  not  only  themselves  not  absorbed,  but,  by  being 
precipitated  and  adhering  to  other  still  undigested  food,  pre- 
vent ready  access  of  enzymes  and  so  greatly  retard  digestion. 

29.  The  intestinal  juice  contains  two  kinds  of  enzymes,  one 
acting  on  protein,  the  other  on  carbohydrate  material.    The 
former  class,  represented  by  the  single  enzyme  erepsin,  has 
no  action  on  the  proteins  of  the  food,  but  splits  peptones  and 
other  products  of  gastric  and  pancreatic  digestion  into  very 


122  THE  HUMAN  MECHANISM 

small  peptids  and  amino-acids.  A  similar  thing  is  true  of  the 
carbohydrate  enzymes  —  they  have  no  action  on  starch  nor  on 
dextrines  (p.  105),  but  disaccharides  (that  is,  sugars  formed 
by  the  chemical  combination  of  two  simple  sugars,  as  dipep- 
tids  are  combinations  of  two  amino-acids)  are  readily  split 
into  their  component  simple  sugars.  Cane  sugar  (sucrose) 
and  milk  sugar  (lactose)  are  two  carbohydrate  foods  which 
belong  to  the  disaccharides ;  a  third  is  maltose,  which  is  the 
stage  in  the  cleavage  of  starch  preceding  the  final  separation 
into  its  component  molecules  of  grape  sugar  (dextrose). 
These  inverting  enzymes  insure  the  complete  cleavage  of  the 
larger  carbohydrate  molecules  into  their  component  sugars, 
precisely  as  erepsin  insures  the  complete  cleavage  of  the 
large  protein  molecule  into  its  component  amino-acids  or 
smaller  peptids. 

Another  most  important  character  of  the  intestinal  juice 
is  its  large  content  of  alkaline  salts,  especially  sodium  car- 
bonate (soda).  Two  processes  constantly  occurring  in  the 
intestine  produce  acid ;  these  are  (1)  the  splitting  of  the 
fats  into  fatty  acids  and  glycerin  by  lipase  and  (2)  the  bac- 
terial decomposition  of  carbohydrates  and  (to  some  extent) 
of  proteins.  The  sodium  carbonate  of  the  intestinal  juice, 
which,  it  will  be  remembered,  is  being  secreted  along  the 
entire  length  of  the  intestine,  neutralizes  these  acids  arid  so 
maintains  the  reaction  of  the  contents  at  an  approximately 
neutral  point.  This  reaction  is  most  favorable  for  the  action 
of  the  enzymes  present.  The  combination  of  sodium  carbon- 
ate with  fatty  acids,  moreover,  forms  soaps,  which  are  more 
readily  soluble  than  the  fatty  acids.  In  this  way  no  doubt 
the  products  of  fat  digestion  are  more  promptly  absorbed 
than  would  otherwise  be  the  case. 

30.  Action  of  the  muscular  coat  of  the  small  intestine. 
The  object  of  the  movements  of  the  intestine  is  not  the 
grinding  down  of  the  food  into  smaller  masses,  but,  in  the 
first  place,  the  agitation  of  the  digesting  mixture  so  that,  on 


ALIMENTATION  AND  DIGESTION 


123 


m 

s  <  s 

M  W  g 
H  O  AH 


o  fa 
3° 
PH 


B| 

8S 

gw 
•^  o 
5  ^ 


Stomach 
Pancreas 
Intestine 


0     c3 

2 


o  o 


02 
1 

'Hb 

e« 

3 


o      ®      CD 

a  s  s 

'-13  '43   .3 


02  CO  02 

CD  CD  O) 

a  s  s 

N3  N  N 

S3     (3  G  S3 

•^      CD  CD  CD 

ft    be  &JD  bC 

O     G  G  S3 


-3 


O 


124 


THE  HUMAN  MECHANISM 


the  one  hand,  good  contact  is  secured  between  food  particles 
and  digestive  juices,  while,  on  the  other  hand,  the  products 
of  digestion  are  quickly  brought  into  contact  with  the  villi  for 
absorption ;  and,  in  the  second  place,  the  slow  movement 
of  the  food  onwards  in  the  intestinal  tube.  To  accomplish 
these  ends  there  are  two  kinds  of  intestinal  movements. 


FIG.  60.   The  divisive,  or  segmenting,  movements  of  the  small  intestine 

A,  surface  view  of  a  portion  of  the  intestine,  showing  six  constrictions  which 
divide  the  contents  into  five  segments,  as  shown  in  B;  as  these  constrictions 
pass  away,  new  ones  come  in  hetween  them  and  divide  each  segment  of  the  con- 
tents into  two,  the  adjoining  halves  of  neighboring  segments  fusing  to  make  the 
new  segments  shown  in  C.  Repetition  of  this  process  results  in  the  condition 

shown  in  D 

31.  Divisive,  or  segmenting,  movements.  The  food  is  not 
distributed  continuously  along  the  entire  length  of  the  intes- 
tine, but  is  subdivided  into  a  number  of  separate  portions 
which  lie  in  different  loops  of  the  tube.  This  is  partly  ex- 
plained by  the  intermittent  character  of  the  discharge  of  the 
chyme  from  the  stomach.  The  number  of  these  portions 
varies  at  different  times,  but  may  be  as  many  as  twenty 
or  even  more.  A  certain  number,  sometimes  all,  of  these 


ALIMENTATION  AND  DIGESTION  125 

masses  of  food  will  be  seen  to  undergo  division  into  small 
segments,  obviously  produced  by  a  series  of  constrictions  of 
the  walls,  as  shown  in  Fig.  60.  The  next  moment  these  are 
replaced  by  a  second  series  of  constrictions  between  the  first. 
Each  segment  is  thus  divided  into  two,  and  the  neighboring 
halves  of  these  segments  fuse.  The  next  moment  the  second 
series  of  constrictions  is  replaced  by  the  first,  and  this  process 
continues  at  times  for  many  minutes  with  no  change  in  the 
general  position  of  the  food  mass.  These  divisive,  or  segment- 
ing, movements  occur  from  twenty  to  thirty  times  a  minute, 
and  it  has  been  estimated  "  that  a  slender  string  of  food 
may  commonly  undergo  division  into  small  particles  more 
than  a  thousand  times  while  scarcely  changing  its  position 
in  the  intestine." 

32.  Peristalsis.  Every  now  and  then  a  ring  of  constriction, 
instead  of  being  confined  to  one  place,  moves  onward,  push- 
ing the  contents  of  the  tube  before  it  for  a  short  distance 
(two  or  more  inches).  A  contraction  of  this  kind  is  called 
peristaltic.  The  effect  produced  is  much  the  same  as  when 
the  contents  of  a  rubber  tube  are  emptied  by  squeezing  it 
along  between  the  thumb  and  finger. 

Thus  each  consignment  of  chyme  delivered  from  the 
stomach  immediately  receives  its  share  of  pancreatic  juice 
and  of  bile,  and  the  final  transformation  of  the  digestible 
foods  takes  place  as  the  whole  is  driven  from  time  to  time 
along  the  intestine  by  peristaltic  contractions,  the  efficiency 
of  the  contact  of  the  food  with  the  digestive  juices,  as  well 
as  its  exposure  to  the  absorbing  surfaces,  being  greatly 
enhanced  by  the  agitation  produced  by  the  movements  of 
constrictive  division  carried  out  by  the  circular  muscles  be- 
tween periods  of  peristaltic  activity.  The  efficiency  of  digestion 
and  absorption  depends  as  much  on  the  movements  carried  out 
by  the  muscular  coat  as  on  the  chemical  processes  effected  by 
enzymes  and  other  constituents  of  the  digestive  juices.  Digestion 
is  always  a  cooperation  of  chemical  and  mechanical  work. 


126 


THE  HUMAN  MECHANISM 


So  far  as  is  known,  these  movements  are  aroused  by  the 
distention  of  the  intestine  with  food  and  possibly  by  chemical 
stimulation  of  the  muscular  coat  by  substances  formed  within 

the  tube.  The  presence  of  solid  in- 
digestible material  also  favors  the 
movements. 

33.  Absorption  is  the  name  given 
to  the  passage  of  digested  food 
materials  from  the  cavity  of  the 
intestine  into  the  blood.  The  word 
itself  perhaps  suggests  that  the 
products  of  digestion  are  received 
into  the  blood  without  change,  as 
a  sponge  might  absorb  a  mixture 
of  peptids,  amino-acids,  sugar,  fatty 
acids,  soaps,  and  inorganic  salts. 
Such,  however,  is  by  no  means  the 
case,  and  the  actual  physical  and 
chemical  processes  of  absorption  are 
complicated  —  far  too  complicated 
to  be  discussed  here.  Suffice  it  to 
say  that  the  intestine  is  not  lined 
by  a  dead  membrane  but  by  living 
cells,  and  through  these  guardian 
cells  the  products  of  digestion  must 
pass  to  enter  the  blood  (see  Fig.  59). 
In  their  passage  through  these  cells 
teal.  Observe  that  the  products  some  of  the  digestive  products  are 

of  digestion  must  first  be  ex-  ..  ,         .      ..  ,,          t, 

acted  upon  chemically  so  that  they 
enter  the  blood  in  forms  more 
available  to  the  tissues  of  the 
body.  The  object  of  the  whole  process  of  alimentation, 
digestion,  and  absorption  would  seem  to  be  that  of  sup- 
plying food  to  the  muscle  fiber,  the  gland  cell,  the  nerve 
cell,  etc.,  through  the  blood  as  an  internal  medium  or 


FIG.  61.  The  intestinal  struc- 
tures concerned  in  absorp- 
tion 

In  one  villas  is  shown  the  close 
network  of  blood  vessels  im- 
mediately under  the  lining 
membrane ;  in  the  other  villus, 
the  central  lymphatic,  or  lac- 


posed    to    absorption    by    the 

blood  vessels  before  they  can 

enter  the  lacteal 


ALIMENTATION  AND  DIGESTION 


127 


middleman,  in  that  form  which  is  best  fitted  for  the  use 
of  the  tissues. 

34.  Digestion  in  the  large  intestine.  The  large  intestine 
contains  no  villi,  and  its  glands  secrete  an  intestinal  juice 
characterized  by  a  large  content  of  mucin  (p.  44). 

In  the  small  intestine  the  amount  of  water  added  by  secre- 
tion balances  that  absorbed,  so  that  the  consistency  of  the 
contents  undergoes  but  little  change  from  the  stomach  to  the 
beginning  of  the  large  intestine.  This  consistency,  it  will  be 


FIG.  62.    The  paths  by  which  the  products  of  digestion  enter  the  general 

circulation 

Those  which  are  absorbed  by  the  blood  vessels  ((7)  of  the  intestine  pass  by  the 
portal  vein  (P.  V.)  to  the  liver  before  they  can  enter  the  right  auricle  (R.A.)  through 
the  hepatic  vein  (H.  V.)  and  the  inferior  vena  cava  (I.  V.C.).  Those  products  which 
are  absorbed  by  the  lacteals  pass  directly  to  the  superior  vena  cava  (S.V.C.) 
through  the  thoracic  duct 

remembered,  was  (approximately)  that  of  moderately  thick 
pea  soup.  During  the  passage  through  the  small  intestine 
the  digested  portions  of  the  food  are  being  removed  by 
absorption,  while  the  indigestible  elements  are  left  behind. 
Among  the  indigestible  elements  of  food  are  certain  connec- 
tive tissues  of  the  animal  foods,  but  especially  the  cellulose 
(p.  97),  which  forms  the  cell  wall  of  plant  tissues.  The  large 
intestine  receives  from  the  small  this  indigestible  material, 
together  with  a  certain  variable  but  usually  comparatively 
small  proportion  of  the  proteins,  fats,  and  carbohydrates 


128 


THE  HUMAN  MECHANISM 


which  have  thus  far  escaped  digestion ;  in  addition  there  are 
certain  constituents  of  the  digestive  juices  which  are  not 
absorbed  and  some  (for  example,  certain  constituents  of  the 
bile)  which  are  distinctly  excretory  products. 

Special  provision  seems  to  be  made  to  insure  the  approxi- 
mately complete  digestion  and  absorption  of  proteins,  carbo- 
hydrates, and  fats  before  the  food  enters  the  large  intestine. 
The  opening  from  the  small  into  the  large  intestine  is  guarded 

by  a  circular  muscle,  the  ileo- 
colic  sphincter,  which  ordinarily 
prevents  the  passage  of  food  out 
of  the  small  intestine  much  as 
the  passage  of  food  from  the 
stomach  is  regulated  at  the 
pylorus  (p.  113).  In  this  man- 
ner considerable  accumulations 
of  material  may  occur  at  the 
end  of  the  small  intestine  and 
remain  there  for  two  hours  or 
more  while  the  combined  ac- 
tion of  enzymes  and  segmenting 
movements  completes  the  diges- 
tion and  absorption  of  the  nu- 
trients. Recent  work  indicates 
that  this  material  is  discharged 
periodically  into  the  large  intestine  by  a  relaxation  of  the 
ileocolic  sphincter  and  a  vigorous  peristalsis  in  the  terminal 
portion  of  the  small  intestine.  It  would  also  seem  that  this 
discharge  is  especially  apt  to  occur  when  food  is  taken  into 
the  stomach,  as  if  there  is  a  reflex  to  this  discharging 
mechanism.  Obviously  the  end  attained  is  the  more  complete 
digestion  of  the  food  in  the  small  intestine. 

Reference  to  Fig.  154  will  show  that  the  large  intestine 
consists  of  four  parts,  the  ascending,  transverse,  and  de- 
scending colons  and  the  rectum,  there  being  an  S-like  bend 


FIG.  63.    Longitudinal  section  of 
the  large  intestine 

Note  the  absence  of  villi 


FIG.  64.    Action  of  the  museular  coat  of  the  large  intestine,  as  shown  by 
the  X-rays.    After  Hertz 

The  lower  border  of  the  ribs  and  the  upper  border  of  the  pelvis  are  sketched. 
Black  shadows  are  the  food  masses  in  the  lower  small  and  the  large  intestine. 
Breakfast  about  7  A.  M.  For  some  time  before  noon  the  food  shadows  showed 
no  change  (12 M).  Shortly  after  12  luncheon  was  taken.  At  12.20  the  food 
accumulated  in  the  lower  small  intestine  had  been  discharged  into  the  ascend- 
ing colon,  which  it  distends.  At  12.23  the  distal  end  of  this  food  mass  was  con- 
stricted off  and  later  (12.25)  passed  along  the  transverse  colon,  where  divisive 
movements  take  place  (12.26) ;  but  at  12.31  the  distal  part  of  this  mass  is  sepa- 
rated and  rapidly  passed  through  the  descending  colon  (12.31  +)  to  the  sigmoid 
flexure  (12.31++) 


129 


130  THE  HUMAN  MECHANISM 

(sigmoid  flexure)  between  the  descending  colon  and  the 
rectum.  The  ascending  colon  is  always  filled,  while  the  rest 
of  the  tube  may  be  empty.  It  is  chiefly  in  this  first  part  of 
the  tube  that  the  abstraction  of  water  occurs.  When,  as  the 
result  of  the  discharge  of  new  material  from  the  small  intes- 
tine into  the  large,  the  ascending  colon  becomes  distended, 
some  of  its  contents  are  pushed  into  the  transverse  colon, 
and  this  material  is  rather  rapidly  passed  by  peristalsis 
through  the  descending  colon,  in  the  lower  part  of  which  it 
accumulates,  being  prevented  from  entering  the  rectum  by 
the  sigmoid  flexure.  Finally,  with  sufficient  accumulation  of 
this  more  solid  material  at  the  sigmoid  flexure,  stronger  peri- 
staltic contractions  move  the  mass  on  into  the  rectum,  which 
thereby  becomes  distended,  and  this  gives  the  desire  to  empty 
the  bowels.  From  this  it  will  be  seen  why  the  bowels  are 
more  readily  emptied  after  meals.  It  is  also  highly  advisable 
to  empty  the  bowels  when  this  desire  comes  on,  since  other- 
wise the  distending  stimulus  loses  its  effectiveness  and  the 
continued  abstraction  of  water  hardens  the  feces. 

35.  Microbic  life  in  the  intestine.  Occurring  simultaneously 
with  the  chemical  changes  produced  by  the  digestive  juices 
are  others  produced  by  microbes  (Part  II),  which  are  always 
found  in  the  intestine  in  large  quantities.  The  acidity  of  the 
gastric  juice  keeps  down  the  numbers  of  these  germs  in  the 
stomach  and,  under  healthy  conditions,  greatly  limits  their 
activity  in  that  organ.  We  have  seen,  however,  that  some 
portions  of  the  contents  of  the  stomach  are  not  acid  in  reac- 
tion during  certain  periods  of  digestion,  and  it  not  infre- 
quently happens  for  this  reason  that  unhealthy  living  and, 
especially,  improper  feeding  may  result  in  serious  gastric 
indigestion  with  excessive  bacterial  decomposition  of  the  food. 
The  production  of  gas,  leading  to  flatulence  or  belching,  is 
one  of  the  most  familiar  results  of  such  bacterial  action. 

In  the  intestine  the  less  strongly  acid  (or  even  neutral  or 
slightly  alkaline)  reaction  is  much  more  favorable  to  bacterial 


ALIMENTATION  AND  DIGESTION  131 

life  and  growth,  and  we  accordingly  find  that  the  number  of 
microbes  is  much  greater  in  the  small  and  large  intestines. 
It  is  not  the  microbe  itself,  however,  which  is  of  importance 
to  the  organism  as  a  whole,  but  the  substances  which  it  pro- 
duces from  the  foods.  Most  of  these  substances  are  either 
harmless  themselves  or  else  are  readily  changed  into  harm- 
less substances  either  before  or  soon  after  entering  the 
blood;  others  are  poisons,  but  are  normally  present  in  such 
minute  quantities  as  to  be  entirely  negligible;  more  rarely 
they  are  produced  in  large  quantities  and  may  cause  various 
ill  effects  either  locally  or  upon  the  body  as  a  whole. 

The  production  of  undue  quantities  of  such  harmful  sub- 
stances, most  of  which  are  derived  from  proteins,  is  chiefly 
dependent  upon  the  food  supply  of  the  bacteria.  This  is 
normally  kept  low  by  the  speedy  and  efficient  removal  of  the 
peptones.  Native1  proteins  are  acted  on  comparatively  slowly 
by  bacteria  and,  in  any  case,  must  first  be  changed  into  pep- 
tones or  simpler  peptids  before  they  can  be  further  broken 
down  into  harmful  bodies.  If,  however,  the  processes  of  absorp- 
tion quickly  and  efficiently  remove  the  digestive  products,  sub- 
sequent harmful  decomposition  of  the  food  is  prevented,  for 
there  are  normally  no  bacteria  in  the  blood.  It  is  therefore  of 
great  importance  to  maintain  the  efficiency  of  absorption.  This 
can  be  done  in  general  only  by  leading  a  normal  life  —  by  tak- 
ing sufficient  muscular  exercise,  by  proper  habits  of  sleep  and 
rest,  by  proper  feeding,  and  so  on.  The  hygienic  conduct  of 
life  tends  to  maintain  all  functions  of  the  body  in  proper  work- 
ing condition,  those  of  the  digestive  organs  included;  and 
nothing  else  can  be  depended  on,  in  the  long  run,  to  do  this. 
To  this  subject  we  shall  return  in  the  chapters  on  hygiene, 
when  dealing  directly  with  the  personal  conduct  of  life. 

1  A  "native"  protein  is  a  protein  as  it  occurs  in  nature  before  being 
changed  by  digestion  or  other  chemical  action.  The  proteins  in  food  are 
largely  native  proteins  or  else,  what  amounts  to  the  same  thing,  as  far 
as  the  action  of  bacteria  is  concerned,  native  proteins  coagulated  by  heat 


132  THE  HUMAN  MECHANISM 

The  chief  seat  of  the  putrefactive  decomposition  of  pro- 
teins is  in  the  large  intestine,  where  conditions  are  favorable 
for  the  activity  of  the  special  bacteria  responsible  for  this 
food  change.  The  reader  will  recall  the  provisions  for  com- 
pleting the  digestion  of  proteins  and  carbohydrates  in  the 
small  intestine,  and  these  certainly  play  a  very  important 
role  in  limiting  harmful  microbic  action  in  the  large  intestine. 
It  often  happens,  especially  in  middle  life,  that  the  quantity 
of  food  eaten,  and  of  protein  food  in  particular,  must  be  con- 
siderably diminished  to  insure  complete  digestion  of  these 
nutrients  in  the  small  intestine  and  thus  deprive  the  putre- 
factive bacteria  of  the  large  intestine  of  the  material  out  of 
which  to  make  deleterious  substances. 

We  have  thus  far  been  dealing  only  with  those  microbes 
commonly  found  in  the  intestine.  At  times  foreign  microbes 
find  entrance,  some  of  which  cause  such  diseases  as  typhoid 
fever,  dysentery,  cholera,  etc.  The  action  of  these  occasional 
intruders  will  be  more  fully  dealt  with  in  Part  II. 

36.  The  elimination  of  intestinal  waste.  Those  who  are 
"  blessed  with  a  good  digestion  "  sometimes  find  it  hard  to 
realize  that  the  preparation  of  food  for  absorption  through  the 
delicate  membranes  lining  the  alimentary  canal  is  a  difficult 
and  complex  process,  requiring  much  delicate  physical  and 
physiological  apparatus  and  involving  various  and  important 
chemical  reactions.  Even  when  they  realize  this,  they  rarely 
appreciate  the  indispensable  cooperation  and  fine  adjustment 
of  the  several  parts  and  processes  concerned.  It  is  just  here, 
however,  that  a  clear  understanding  is  important,  for  without 
this  it  is  not  easy  to  see  how  disorders  of  digestion  arise. 

Let  us  then  remember  that  the  efficient  handling  of  the  food 
in  the  stomach  is  aided  by  the  preparatory  crushing  it  receives 
in  the  process  of  mastication  ;  that  in  the  stomach  an  adequate 
and  efficient  secretion  of  gastric  juice  must  take  place,  and  that 
this  begins  as  the  result  of  nervous  events  connected  with 
our  enjoyment  of  the  food  when  eaten ;  that  the  continued 


ALIMENTATION  AND  DIGESTION  133 

secretion  of  gastric  juice  is  secured,  in  turn,  by  stimulation  of 
the  mucous  membrane  of  the  stomach  by  the  peptones  which 
the  psychic  secretion  has  formed  from  the  proteins  of  the  food  ; 
and,  finally,  that  the  chemical  action  of  the  gastric  juice  is 
aided  by  the  peculiar  contractions  of  the  muscular  coat  of  the 
stomach.  All  these  agencies  working  together  deliver  the  food 
to  the  intestine  in  a  finely  divided  state,  well  adapted  and  in- 
deed absolutely  necessary  to  secure  the  proper  contact  of  the 
food  with  the  pancreatic  juice,  the  bile,  and  the  intestinal  juice. 

The  flow  of  pancreatic  juice,  in  turn,  is  partly  the  result 
of  the  action  of  the  hydrochloric  acid  of  the  chyme  on  the 
walls  of  the  intestine,  while  the  efficiency  of  the  action  of  the 
pancreatic  enzymes  depends  upon  the  simultaneous  action  of 
the  bile  and  the  intestinal  juice ;  lastly,  the  chemical  action 
of  these  juices,  as  well  as  the  final  act  of  absorption,  requires 
the  cooperation  of  the  muscular  coat.  Healthy  conditions 
with  respect  to  bacterial  action  similarly  depend  upon  all 
else  occurring  as  it  should.  Digestion,  in  short,  is  a  chain  of 
events,  each  depending  upon  those  which  have  gone  before 
and,  to  a  large  extent,  upon  those  which  are  taking  place  at 
the  same  time. 

Keeping  these  facts  in  mind,  it  is  easy  to  appreciate  the 
possibility  of  diarrhea  or  constipation,  the  latter  consisting 
in  the  retention  of  wastes,  the  poisonous  constituents  of  which 
may  be  absorbed  into  the  body  and  cause  discomfort,  head- 
aches, and  malaise.  When  all  the  digestive  processes  work 
together  properly  there  should  be  a  perfectly  natural  and 
regular  evacuation  of  the  bowels.  The  frequency  of  such 
evacuation  varies  somewhat  and  is  largely  a  matter  of  habit ; 
with  some 'people  it  is  twice  a  day,  with  others  once  every 
other  day,  but  with  the  vast  majority  it  is  normally  once  every 
day  and  at  about  the  same  time.  Where  this  is  not  the  case 
there  is  reason  to  believe  that  some  part  of  the  work  of  diges- 
tion is  not  being  properly  performed.  The  trouble  is  not 
ordinarily  in  the  mechanism  governing  the  actual  discharge 


134  THE  HUMAN  MECHANISM 

of  the  feces  from  the  rectum,  but  in  a  derangement  some- 
where else;  it  may  be  entirely  the  fault  of  the  mechanism 
of  peristalsis,  or  it  may  be  due  to  imperfect  secretion.  In  all 
cases  it  means  that  something  is  wrong,  &nd  .the  remedy  should 
be  sought  not  in  drugs  or  pills  but  in  search  for  and  removal 
of  the  cause.  A  moment's  consideration  will  show  the  reason- 
ableness of  this  position.  If  a  watch  loses  time  because  it 
needs  cleaning,  we  do  not  seek  a  remedy  in  drugs,  but  in  its 
cleaning,  better  adjustment,  and  good  care ;  and  the  remedy 
for  diarrhea  or  constipation  should  in  all  cases  be  sought  for 
in  the  better  conduct  of  life.  Is  enough  muscular  exercise 
being  taken?  Is  the  diet  properly  chosen?  Are  we  drinking 
enough  water?  Especially,  is  the  food  of  sufficient  bulk  and 
does  it  contain  enough  laxative  material  (such  as  fruit)? 
Above  all,  are  we  getting  enough  sleep  ?  Are  we  over- 
working, or  do  we  work  too  long  at  a  time  without  resting? 
Is  our  clothing  warm  enough,  or  are  we  overclad?  Such  are 
the  questions  which  should  be  seriously  asked.  The  student 
of  personal  hygiene  cannot  lay  to  heart  too  seriously  the 
truth  that  the  man  who  goes  from  day  to  day,  from  week 
to  week,  from  year  to  year,  neglecting  the  warnings  of  diar- 
rhea or  constipation,  only  reaps  the  harvest  of  his  folly  when 
in  later  years  he  suffers  loss  of  health  and  at  times  bodily 
discomfort;  and  it  is  nothing  short  of  impiety  to  marvel 
under  such  circumstances  at  the  "  mysterious "  ways  of  a 
Providence  which  so  "  afflicts "  his  creatures.  It  is  no  ex- 
aggeration to  say  that  the  regular  discharge  of  the  wastes  is 
quite  as  important  as  the  regular  feeding  of  the  body  and 
that  no  less  pains  should  be  taken  to  form  good  habits  in  the 
one  case  than  in  the  other.  Many  of  the  headaches,  many  of 
the  bad  feelings,  and  many  of  the  bad  tempers  of  the  world 
are  due  to  neglect  of  this  simple  fact.  No  city,  however  well 
fed  or  beautiful,  the  drains  of  which  are  choked  with  filth, 
can  long  remain  either  wholesome  or  attractive  —  and  the 
human  body  is  essentially  a  city  teeming  with  living  cells. 


CHAPTER  IX 

THE  CIRCULATION  OF  THE  BLOOD 
A.  BLOOD  AND  LYMPH 

1.  The  blood  as  a  common  carrier.  In  previous  chapters 
some  of  the  more  general  features  of  the  circulation  have 
already  been  touched  upon.  In  studying  the  parts  of  the 
body  the  student  has  become  somewhat  acquainted  with  the 
heart,  the  arteries,  and  the  veins;  in  considering  the  typical 
structure  of  the  organs  (Chap.  Ill)  he  has  seen  how  the 
arteries  are  connected  with  the  veins  by  a  system  of  com- 
municating tubes,  the  capillaries,  through  the  thin  walls  of 
which  interchange  takes  place  between  the  lymph  and  the 
blood ;  and  in  studying  the  interdependence  and  cooperation 
of  the  cells  and  organs  (Chap.  VI)  he  has  learned  how  the 
blood  leaving  each  organ  returns  to  the  heart,  there  to  be 
mixed  with  that  coming  from  all  other  organs  and  thence 
pumped  first  to  the  lungs  and  then  to  the  rest  of  the  body. 
The  need  of  a  circulation  is  obvious,  for  the  food  received 
from  the  alimentary  canal  and  the  oxygen  received  from  the 
lungs  must  somehow  be  carried  to  the  muscle  fibers,  the 
nerve  cells,  the  gland  cells ;  the  cellular  wastes  must  be 
taken  away  to  the  organs  of  excretion ;  and  the  internal 
secretions  of  the  body  must  be  transported  from  the  organs 
in  which  they  are  made  to  those  in  which  they  are  to  be 
used.  In  other  words,  it  is  a  necessary  corollary  to  the  fact 
that  no  cell  or  organ  "  liveth  unto  itself "  that  there  should 
be  some  common  carrier  of  matter  and  of  energy  from  one 
organ  to  another.  Such  a  common  carrier  is  the  blood.  The 
analogy  of  the  blood  system  of  the  body  with  the  railway 

135 


136 


THE  HUMAN  MECHANISM 


system  of  a  country  is  instructive.  As  different  persons  and 
different  communities  in  any  country  make  different  prod- 
ucts and  have  different  needs,  it  becomes  more  and  more 
necessary  that  the  means  of  communication  between  them 

be  extensive  and  efficient. 
Hence  the  remarkable 
growth  of  the  railroads, 
or  "  common  carriers,"  of 
any  country  in  which  in- 
dustrial development  pro- 
duces increasing  division 
of  labor. 

The  blood,  which  is  thus 
the  common  carrier  first 
between  the  various  or- 
gans and  second  between 
each  organ  and  the  outer 
environment,  is  the  net 
product  of  the  united 
work  of  all  the  organs: 
from  the  alimentary  canal 
it  receives  water  and  the 
products  of  digestion ; 
from  the  lungs  it  receives 
oxygen ;  each  organ  con- 
tributes its  share  of  waste 
products  or  of  internal 
secretion,  while  some  in- 
fluence the  composition  of 
the  blood  by  removing 
from  it  certain  things  that  it  contains. 

2.  The  microscopic  structure  of  the  blood.  Examined  under 
the  microscope  the  blood  is  seen  to  consist  of  a  liquid  portion, 
the  plasma,  crowded  with  small  solid  bodies,  the  corpuscles. 
These  are  of  two  kinds :  the  red  corpuscles  —  biconcave  disks 


FIG.  65.    Structure  of  a  drop  of  blood  as 
seen  under  the  microscope 

Above  are  shown  nine  red  corpuscles  highly 
magnified ;  below,  less  highly  magnified, 
the  appearance  of  the  blood  soon  after  being 
drawn.  Two  white  corpuscles  are  shown, 
and  the  red  corpuscles  stick  together,  form- 
ing "  rouleaux."  Size  of  red  corpuscle,  7.7 n 
wide,  2-4 /^  thick.  Diameter  of  white  cor- 
puscle, 5-10  M.  Number  of  red  corpuscles, 
4,500,000-5,000,000  per  cubic  millimeter; 
number  of  white  corpuscles,  4500-13,000 
per  cubic  millimeter,  according  to  the  state 
of  digestion,  etc.  Surface  area  of  all  the 
red  corpuscles  of  the  blood,  3000  square 
meters  (30,000  square  feet  or  approximately 
four  times  the  size  of  a  baseball  diamond). 
(!M,  or  micron  —  0.001  millimeter) 


THE  CIRCULATION  OF  THE  BLOOD 


137 


containing  a  pigment,  hemoglobin,  which  gives  the  red  color 
to  the  blood ;  and  the  white  corpuscles,  which  are  colorless, 
nucleated  cells. 

Important  data  on  the  number,  size,  and  surface  area  of 
the  corpuscles  will  be  found  in  connection  with  Fig.  65. 

3.  The  white  blood  corpuscles.  The  white  blood  corpuscles 
really  comprise  several  different  kinds  of  cells,  having  differ- 
ent functions,  the  study  and  explanation  of  which  belong  to 
advanced  rather  than  to 
elementary  physiology.  It 
is  enough  for  our  purpose 
to  state  that  these  cells  are 
not  confined  to  the  blood, 
but  work  their  way  out  of 
the  blood  vessels  between 
the  cells  of  the  capillary 
walls  and  are  often  found 
in  the  lymph  spaces  of  the 
tissues  as  wandering  cells. 
The  latter  term  refers  to 
their  movement  from  place  FIG.  66.  Amoeboid  movement  of  a  white 
to  place.  The  cytoplasm  of  corpuscle 

the    white     Corpuscle    is    a      Showing  four  consecutive  positions  among 
.  .  ,  (.    .  ,       .  .  a  group  of  red  corpuscles 

thick,  viscous  fluid  without 

constant  or  definite  form.  In  locomotion  the  cytoplasm  flows 
slowly  from  some  part  of  the  surface  in  the  direction  of 
motion,  forming  what  is  known  as  a  pseudopodium  (from  the 
Greek,  meaning  a  false  foot),  as  shown  in  Fig.  66 ;  the  rest 
of  the  body  of  the  corpuscle  then  flows  into  the  pseudo- 
podium.  By  the  continuation  of  this  process  the  white  cor- 
puscles make  their  way  through  the  spaces  of  the  connective 
tissue.  Locomotion  by  means  of  pseudopodia  is  frequently 
spoken  of  as  amoeboid,  from  the  amoeba,  a  unicellular  animal 
which  moves  in  the  same  manner.  (See  Chapter  XXIII  for 
examples  of  the  functions  of  white  blood  corpuscles.) 


138      •  THE  HUMAN  MECHANISM 

4.  The  red  blood  corpuscles.    The  red  corpuscles  are  pig- 
mented,  biconcave  disks  with  no  nucleus ;  they  are  normally 
confined  to  the  blood  vessels  and  are  carried  around  passively 
in  the  blood  current  without  active  movements  of  their  own. 
The  main  function  of  these  corpuscles  is  to  carry  oxygen 
from  the  lungs  to  the  tissues,  a  function  which  will  be  fur- 
ther studied  in  connection  with  respiration.    They  contain  a 
pigment,  hemoglobin,  which  gives  to  the  blood  its  red  color 
and  carries  the  oxygen. 

5.  The    blood    plasma    is    an    exceedingly    complex    fluid 
whose  general  composition  is  represented  as  follows:  water, 
90  parts;  solids,  10  parts  (proteins,  8  parts;  inorganic  salts, 
1  part;  extractives,  1  part). 

Under  the  extractives  are  included  a  very  large  number 
of  substances  which,  though  present  in  small  quantities,  are 
interesting  to  the  physiologist  because  they  are  largely  prod- 
ucts of  the  chemical  activities  of  the  body  and  as  such 
give  information  about  the  nature  of  the  chemical  changes 
occurring  in  the  organs. 

Finally,  it  should  be  remembered  that  the  cells  of  the 
body  generally  are  bathed  with  lymph,  not  with  blood ; 
in  other  words,  that  the  lymph  and  not  the  blood  is  the 
immediate  environment  of  the  cells.  Lymph  is  sometimes 
described  as  blood  minus  its  red  corpuscles;  but  this  state- 
ment, though  convenient,  is  not  strictly  correct,  since  the 
amount  of  waste  products  in  lymph  must  be  greater  than 
in  blood,  while  the  amount  of  food  material  must  be  less 
(see  Chap.  IV).  Much  as  the  blood  is  a  product  of  the 
united  chemical  activity  of  all  the  organs  of  the  body,  so 
the  lymph  of  each  organ  is  derived  from  the  cells  of  that 
organ  and  from  the  blood  flowing  through  it.  Lymph  thus 
has  a  double  origin  and  of  course  shows  very  considerable 
differences  of  composition  in  different  organs. 


THE  CIRCULATION  OF  THE  BLOOD 


139 


B.  MECHANICS  OF  THE  CIRCULATION  OF  THE  BLOOD 

AND   OF  THE   FLOW   OF   LYMPH 

The  greatest  discovery  ever  made  in  physiology  was  that 
of  the  circulation  of  the  blood.  As  late  as  the  settlement  of 
the  earliest  English  colonies  in  America  it  was  thought  that 
the  blood  moved  back  and  forth  in  the  blood  vessels,  as  the 
waters  in  the  sea  ebb  and  flow ;  but  of  any  circulation,  in  the 
sense  of  a  steady  stream  returning  to  its  source,  there  was 
no  idea;  and  it  was  not  until  1621  that  William  Harvey,  an 
English  physician,  proved  be- 
yond the  shadow  of  a  doubt 
that  the  blood  in  the  body  of 
all  the  higher  animals  flows  like 
a  stream  always  in  one  direc- 
tion, ultimately  returning  to  its 
source. 

Tests  made  upon  various 
animals  have  shown  that  this 
circulation  is  accomplished  in 
the  surprisingly  short  time  of 
from  twenty  to  thirty  seconds ; 
which  means  that  the  whole 
mass  of  the  blood  (in  man  about  twelve  pints)  passes  between 
three  and  four  thousand  times  a  day  through  the  various 
organs  of  the  body,  bringing  to  them  their  food,  carrying 
away  their  wastes,  and  in  general  helping  to  maintain  normal 
conditions.  By  what  hydraulic  machinery  is  this  marvelous 
work  done  ? 

6.  The  motive  power  of  the  circulation  as  a  whole ;  the  beat 
of  the  heart.  Whenever  a  mass  of  liquid  is  kept  in  motion 
we  naturally  look  first  for  the  motive  power.  In  answering 
the  question,  What  makes  the  blood  circulate  ?  we  shall 
find  that  while  there  are  several  causes,  one  of  these,  namely 
the  beat  of  the  heart,  is  vastly  more  important  than  all  the 


FIG.  67.    The  circulation  of  the 

blood  as  seen  in  the  small  arteries 

and  capillaries  of  the  web  of  a 

frog's  foot 


140  THE  HUMAN  MECHANISM 

others  combined.  This  fact  is  now  so  familiar  that  it  is  hard 
to  realize  that  we  owe  to  Harvey  not  only  the  discovery  of 
the  circulation  but  also  the  discovery  of  the  meaning  of  the 
heart  beat.  Before  his  time,  to  be  sure,  the  living  heart  had 
been  seen  at  work,  alternately  shrinking  in  size  and  then 
swelling,  the  shrinking  being  called  systole  and  the  swelling 
diastole ;  but  these  changes  in  size  were  regarded  as  the 
results  of  the  contraction  and  expansion  of  certain  "vital 
spirits  "  which  the  arterial  blood  was  then  supposed  to  contain, 
and  not  as  muscular  contractions  and  relaxations.  Harvey 
showed  that  the  heart  is  a  powerful  muscle  and  that  its  systole 
is  a  muscular  contraction ;  that  during  systole  it  becomes 
hard,  just  as  the  biceps  muscle  does  when  it  shortens,  and 
during  diastole  soft  and  flabby;  he  also  proved  that  with 
each  systole  the  heart  drives  or  spouts  blood  into  the  large 
arteries  (the  aorta  and  the  pulmonary  artery),  and  that  this 
blood  is  prevented  from  flowing  back  into  the  heart  during 
diastole  by  membranous  valves  at  the  very  beginning  of  the 
large  arteries  in  question. 

7.  The  heart  a  muscular  force  pump.  The  beat  of  the 
heart,  even  to  its  most  minute  detail,  is  one  of  the  most 
important  as  well  as  one  of  the  most  interesting  subjects  in 
physiology;  everything  in  the  body  hangs  on  its  proper  effi- 
ciency and  regulation,  and  it  cannot  be  too  thoroughly 
studied.  For  our  present  purposes  it  will  suffice  to  describe 
the  heart  as  composed  essentially  of  a  pair  of  muscular  force 
pumps.  Dissection  shows  that  it  is  divided  into  right  and 
left  halves  (see  Fig.  70),  completely  separated  from  each 
other,  and  that  each  half  consists  of  two  chambers  —  an 
auricle  and  a  ventricle.  The  auricles,  into  which  the  great 
veins  open,  have  thin  muscular  walls  and  are  comparatively 
small  in  size ;  the  ventricles,  on  the  other  hand,  from  which 
the  great  arteries  arise,  have  thick  muscular  walls,  especially 
the  left  ventricle.  The  ventricles,  indeed,  constitute  the  prin- 
cipal part  of  the  force  pump ;  the  auricles  merely  facilitate 


THE  CIRCULATION  OF  THE  BLOOD 


141 


the  work  of  the  ventricles  and  for  purposes  of  elementary 
study  may  be  mostly  neglected.  The  student  should,  if  pos- 
sible, examine  for  himself  and  actually  handle  the  auricles, 
ventricles,  and  great  blood  vessels  of  a  sheep's  heart,  which 
in  size  and  structure  sufficiently  resembles  the  human  heart. 
Figs.  15  and  162  should  also  be  consulted. 

8.  The  mechanics  of  the  heart  beat.  All  force  pumps  con- 
sist of  two  indispensable  parts  —  some  device  for  pressing 
upon  a  liquid  within  a  chamber,  and  valves  at  the  openings 
of  the  chamber  so  arranged  as  to  allow  the  passage  of  the 
liquid  in  one  direction 
only.  Each  ventricle  of 
the  heart  is  really  such 
a  pump  and  is  pro- 
vided with  two  sets  of 
valves  —  one  set  at  the 
inlet,  between  the  auri- 
cles and  the  ventricles, 
and  the  other  at  the 
arterial  outlet.  These 
valves  permit  blood  to 
pass  only  from  the 
great  veins  through  the 

auricles  and  011  through  the  ventricles  to  the  great  arteries. 
The  contraction  of  the  muscular  wall  of  the  ventricles  pro- 
duces pressure  on  the  blood  within  their  cavities ;  this 
pressure  quickly  and  easily  closes  the  auriculo-ventricular 
valves  and  finally  forces  open  the  shut  valves  at  the  open- 
ings of  the  great  arteries.  In  this  way  the  right  ventricle 
drives  venous  blood  into  the  pulmonary  artery,  and  the  left 
ventricle  arterial  blood  into  the  aorta.  With  the  relaxa- 
tion of  the  ventricles  (diastole)  pressure  falls  within  their 
cavities,  and  were  it  not  for  the  valves  at  the  mouths  of 
the  aorta  and  the  pulmonary  artery,  blood  would  regurgitate, 
or  flow  back,  into  the  heart ;  but  this  "  slip "  (as  it  is 


FIG.  68.    Diagram  of  the  action  of 
a  force  pump 


142 


THE  HUMAN  MECHANISM 


called  in  hydraulics)  the  valves  prevent,  and  the  ventricles 
again  fill  through  the  only  open  channel,  that  is,  the  one 
leading  from  the  great  veins  and  the  auricles.  Thus  by 
contractions  rhythmically  repeated  the  heart  continues  to 
spout  or  deliver  blood  from  the  two  sets  of  great  veins  into 
the  two  sets  of  great  arteries.  It  is  plainly  a  double  force 
pump  or,  better,  a  pair  of  force  pumps  lying  and  working 
side  by  side. 

9.  The  arterial  and  the  venous  reservoirs.  To  understand 
the  exact  nature  and  result  of  the  work  of  the  heart  we  must 

now  consider  the  rela- 
tion of  this  living  pump 
to  the  pipe  system  (ar- 
teries, capillaries,  and 
veins)  with  which  it 
is  connected.  The  stu- 
dent should  first  trace 
the  general  course  of 
the  circulation  in  the 
simple  diagrammatic 
representation  given  in 
Fig.  70.  This  shows 
that  the  blood  which 
enters  the  aorta  from 
the  left  ventricle  must 
return  to  the  right  side  of  the  heart  and  pass  through  the 
lungs  before  it  can  again  reach  the  aorta.  As  the  physical 
principles  of  the  circulation  are  the  same  for  the  systemic 
and  the  pulmonary  vessels,  we  shall  confine  our  attention 
to  the  former. 

In  the  first  place,  we  may  observe  that  the  heart  pumps  the 
blood  into  what  is  practically  a  large  reservoir  (the  larger 
arteries)  and  that  the  blood  flows  from  this  reservoir  to  a  sec- 
ond reservoir  (the  larger  veins)  by  various  routes ;  for  the  ves- 
sels of  the  different  organs  represent  many  alternative  courses 


FIG.  69.  The  force-pump  action  of  a  ventricle 
of  the  heart 

On  the  left  is  shown  the  condition  during  dias- 
tole; on  the  right,  during  systole 


THE  CIRCULATION  OF  THE  BLOOD 


143 


which  the  blood  may  take 
in  flowing  from  the  arte- 
rial to  the  venous  reser- 
voir. The  blood  stream, 
indeed,  may  be  compared 
with  a  stream  supplying 
water  power  to  a  series  of 
mills  in  a  manufacturing 
town.  The  larger  arteries 
from  the  main  source  of 
pressure  (the  heart)  cor- 
respond to  the  headrace 
from  above  the  dam,  while 
the  larger  veins  correspond 
to  the  tailrace.  The  water 
flows  from  the  one  into 
the  other  only  through  the 
smaller  sluices,  or  pen- 
stocks, which  supply  the 
mills.  So  in  the  vascular 
system  a  part  of  the  blood 
pumped  into  the  arterial 
reservoir,  or  aorta,  finds 
its  way  into  the  venous 
reservoir  by  way  of  the 
skin,  another  part  by  way 
of  the  digestive  organs, 
another  by  way  of  the 
brain,  still  another  by 
way  of  the  kidneys,  and 
so  on ;  but  the  flow  in 
every  case  is  essentially 
the  same,  namely  from  a 
reservoir  of  high  pressure 
to  one  of  lower  pressure. 


FIG.  70. 

Diagram 

of  the  organs 

of  the  circulation 

L,  pulmonary  circulation;  M,  circulation 
through  the  organs  suspended  by  the  mes- 
entery, the  blood  being  carried  to  the  liver 
P  before  it  returns  to  the  heart.  The  circu- 
lation through  other  organs,  such  as  brain, 
muscles,  skin,  and  kidneys,  is  indicated. 
Lymphatics  are  represented  by  dotted  lines 


144  THE  HUMAN  MECHANISM 

10.  The  driving  force  for  the  flow  of  blood  from  the  aorta ; 
pressure  in  arteries  and  veins.  The  hydraulic  conditions  in 
the  aorta  may  be  illustrated  by  means  of  the  following  sim- 
ple piece  of  apparatus :  To  an  ordinary  rubber  syringe  attach 
a  piece  of  elastic  rubber  tubing,  the  other  end  of  which  is 
closed  by  a  detachable  nozzle.  If  now  the  nozzle  be  removed 
and  water  pumped  into  the  tube,  it  will  be  found  that  the 
flow  from  the  open  end  consists  of  squirts  or  spouts  and 
continues  only  during  the  stroke  of  the  pump;  if,  however, 
we  attach  the  nozzle  and  again  pump  water  into  the  .tube, 
the  resistance  caused  by  the  small  orifice  of  the  nozzle  pre- 
vents the  water  from  flowing  out  of  the  tube  as  fast  as  the 
syringe  pumps  it  in.  The  tubing  becomes  distended  with  water. 
Since,  however,  the  tube  is  elastic,1  and  so  tends  to  return 
to  its  original  size,  it  forces  the  liquid  out  through  the 
nozzle  even  between  the  strokes  of  the  pump.  The  imme- 
diate cause  of  the  steady  flow  from  the  nozzle  is  therefore 
the  elastic  squeeze  of  the  rubber  tube.  The  intermittent 
stroke  of  the  pump  produces  distention  of  the  tube,  and  the 
elasticity  of  the  distended  tube  constantly  forces  the  water 
out  of  the  nozzle. 

Closely  similar  conditions  obtain  in  the  arterial  reservoir. 
Here  the  outlet  is  also  through  very  small  tubes,  the  small 
arteries,  whose  bore  is  not  greater  than  -^  or  yl ^  of  an  inch ; 
which  fact  introduces  the  same  condition  as  does  the  nozzle 
of  our  apparatus,  that  is,  a  resistance  to  the  outward  flow  of 
the  blood.  Consequently  the  blood  cannot  flow  out  of  the 
aorta  as  rapidly  as  it  is  driven  in,  and  the  extensible  and 
elastic  walls  are  necessarily  stretched.  The  immediate  effect 

1  An  elastic  body  is  one  which  returns  to  its  original  shape  when  it  has 
been  stretched,  compressed,  or  otherwise  deformed.  Elasticity  must  not  be 
confounded  with  "  extensibility,"  or  the  property  of  allowing  stretching. 
Thus  when  we  "pull"  taffy  we  deal  with  a  body  which  is  very  extensible 
but  which  is  practically  inelastic.  A  body,  indeed,  may  be  extensible  only 
with  difficulty,  but  possess  a  very  high  degree  of  elasticity ;  ivory  is  a  good 
example  of  this  kind. 


(Esophagus 


To  head  and 
neck 

To  shoulder  and 
arm 


To  stomach, 
intestine,  etc. 

To  kidney 


To  leg 


FIG.  71.   The  aorta  and  its  main  branches 

At  the  beginning  are  shown  the  three  pocket  valves  which  prevent  regurgitation 
of  blood  during  diastole 


145 


146  THE  HUMAN  MECHANISM 

of  the  heart  beat  is  to  keep  the  arterial  reservoir  overfilled 
or  distended,  so  that  the  elastic  reaction  of  its  walls  is  brought 
into  play;  and  it  is  this  elastic  reaction  of  the  arterial  walls 
which  is  the  immediate  cause  of  the  steady  outflow  through 
the  small  arteries  and  capillaries. 

The  force  of  compression,  or  pressure,  exerted  by  the 
clastic  arterial  walls  is  primarily  exerted  upon  the  blood 
within  them ;  and  the  more  the  arteries  are  distended  the 
greater  will  be  the  pressure  exerted  on  the  blood.  A  liquid 
thus  under  pressure  tends  to  find  an  outlet;  should  any 
part  of  the  arterial  wall  be  weak,  as  sometimes  happens  in 
diseased  conditions,  it  is  bulged  outward ;  and,  for  the  same 
reason,  a  flow  of  blood  will  take  place  through  such  outlets  as 
are  presented  by  the  smaller  arteries  and  capillaries.  More- 
over, the  greater  the  pressure  of  the  blood  in  the  arteries,  the 
more  rapid  will  be  the  flow  into  the  capillaries.  Hence  it  is 
customary  to  use  the  arterial  blood  pressure  as  a  measure  of 
the  force  of  elasticity  exerted  by  the  distended  arterial  wall. 

The  veins,  on  the  other  hand,  are  less  elastic  than  the 
arteries ;  they  are,  indeed,  more  like  mere  conducting  tubes 
through  which  the  blood  can  flow  back  to  the  heart.  They 
are  not  overfilled  (since,  for  one  reason,  there  is  no  resistance 
to  the  flow  of  blood  out  of  them  into  the  heart)  and  hence 
venous  blood  pressure  is  low. 

Thus  we  have  the  conditions  favorable  for  the  flow  from 
the  aorta  to  the  great  veins  —  a  high  pressure  in  the  arte- 
rial reservoir  and  a  low  pressure  in  the  venous  reservoir. 
It  is  the  function  of  the  heart,  by  continually  pumping  the 
blood  from  the  veins  into  the  arteries,  to  keep  the  arterial 
reservoir  distended,  thus  maintaining  a  difference  of  pressure 
in  the  two  reservoirs.  It  is  this  difference  of  pressure  which 
drives  the  blood  through  the  organs. 

11.  The  distribution  of  the  blood  among  the  organs.  Some 
organs  require  more  blood  than  others,  and  the  same  organ 
often  requires  more  blood  at  one  time  than  at  another.  Thus 


THE  CIRCULATION  OF  THE  BLOOD  147 

muscles  and  glands,  the  seat  of  very  active  chemical  changes, 
require  more  blood  than  a  tendon ;  and  a  gland  requires 
more  blood  during  the  process  of  secretion  than  during  rest. 
How  is  the  supply  of  blood  to  the  organs  regulated  to  meet 
their  varying  needs  ?  In  the  first  place,  some  organs  are 
more  vascular  than  others;  those  requiring  a  larger  supply 
of  blood  receive  a  greater  number  of  arteries  from  the  arte- 
rial reservoir  and  have  a  closer  network  of  capillaries.  But 
in  addition  to  this,  these  smaller  arteries  contain  circular 
muscle  fibers  whose  contraction  diminishes  the  bore  of  the 
tube.  When  an  organ  needs  more  blood  the  muscle  fibers  of 
its  small  arteries  relax,  thus  permitting  the  arterial  tubes  to 
widen  or  dilate  —  just  as  when  we  want  the  water  to  flow 
faster  from  a  faucet,  we  widen  the  outlet  from  the  pipe  by 
turning  the  spigot  a  little  further.  When  less  blood  is  needed 
the  small  arteries  are  caused  to  constrict,  just  as  a  spigot 
may  be  partially  turned  off  (see  sects.  25-27).  In  this  way 
the  flow  of  blood  to  any  organ  is  regulated  to  meet  the 
varying  needs  of  the  organ  in  question.1 

12.  Secondary  aids  to  the  circulation.  In  the  preceding 
discussion  we  have  seen  that  the  cause  of  the  flow  of  blood 
through  the  organs  is  the  difference  of  pressure  in  the  two 
reservoirs.  We  have  further  seen  that  this  difference  of 
pressure  is  maintained  by  the  heart  beat  in  pumping  blood 
from  the  venous  into  the  arterial  reservoir.  A  moment's 
consideration  will  show  that  anything  which  hastens  the  flow 
of  blood  from  the  veins  into  the  heart  and  so  lowers  pres- 
sure within  the  veins  would  similarly  aid  the  circulation, 

1  In  order  that  the  student  may  become  more  familiar  with  these  funda- 
mental hydraulic  principles  of  the  circulation,  such  questions  as  the  follow- 
ing should  be  answered  :  (1)  What  are  the  two  principal  factors  whose 
variations  change  the  amount  of  arterial  pressure  ?  Illustrate  by  an  example 
or  model.  (2)  How  would  the  dilation  of  all  the  arteries  of  the  intestine 
affect  the  general  arterial  pressure  ?  (3)  What  would  be  the  effect  upon  the 
amount  of  blood  flowing  through  the  skin  under  this  condition  ?  (4)  How 
would  dilation  of  the  arteries  of  the  skin  affect  the  blood  flow  through 
the  brain? 


148 


THE  HUMAN  MECHANISM 


since,  with  the  same  arterial  pressure,  more  blood  will  flow 

into  an  empty  vein  than  into  one  which  is  partially  filled. 

1.   The  breathing  movements.    There  are  two  factors  which 

thus  tend  to  empty  the  veins.    The  first  is  the  suction  exerted 

on  the  blood  within  the  veins 
by  breathing  movements.  The 
exact  mechanism  by  which 
this  is  accomplished  must 
be  left  for  consideration  in 
the  chapter  on  respiration. 
Suffice  it  to  say  here  that 
just  as  the  enlargement  of 
the  thorax,  when  we  take 
in  a  breath,  sucks  air  into 
the  lungs,  so  it  also  sucks 
blood  from  the  large  veins 
outside  the  thorax  into  those 
which  lie  within  it ;  because 
of  the  thickness  of  the 
walls  of  the  arteries  the 
same  effect  occurs  to  only 
a  very  slight  extent  in  the 
arterial  reservoir.  During 
expiration,  on  the  other 
hand,  the  reduction  in  size 
of  the  thorax  forces  air  out 
of  the  lungs,  arid  we  might 
expect  that  it  would  simi- 
larly force  blood  from  the 

veins  within  the  thorax  into  those  without.  And  this  it  cer- 
tainly would  do  if  the  veins  were  not  provided  with  valves 
which  allow  the  blood  to  flow  only  toward  the  heart.  In 
general,  therefore,  both  inspiration  and  expiration  aid  the  cir- 
culation, the  former  by  sucking  blood  into  the  thoracic  veins 
and  so  emptying  those  outside,  the  latter  by  making  this 


FIG.  72.    Cross  sections  of  portions  of 

the  wall  of  a  smaller  artery  (a)  and  a 

smaller  vein  (») 

A,  internal  coat;  B,  middle  coat,  with 
muscle  fibers ;  C,  outer  coat  of  connective 
tissue.  The  contraction  of  the  circularly 
disposed  muscle  fibers  narrows  the  bore 
of  the  tube 


THE  CIRCULATION  OF  THE  BLOOD 


149 


blood  in  the  intrathoracic  veins  flow  on  more  rapidly  to  the 
heart,  whence  it  is  pumped  into  the  arteries.  In  a  word, 
deep  breathing  greatly  promotes  a  good  circulation. 

2.  Intermittent  compression  of  the  veins.  The  other  second- 
ary factor  of  the  circulation  is  intermittent  compression  of  the 
veins,  and  in  ordinary  life  this  is  brought  about  in  two  ways  : 
(1)  Whenever  a  muscle  contracts  it  thickens  and  hardens; 
the  veins  and  capilla- 
ries which  are  between 
the  fibers  and  fiber  bun- 
dles, or  in  the  con- 
nective tissue  between 
two  contracting  mus- 
cles, will  thus  have  the 
blood  squeezed  out  of 
them  into  the  large 
veins ;  when  the  mus- 
cle relaxes,  the  empty 
veins  and  capillaries 
will  readily  fill  from 
the  arteries,  since  the  valves  of  the  veins  will  prevent  any 
backward  flow  of  the  blood  from  the  larger  veins.  Alternate 
contractions  and  relaxations  of  muscles  therefore  aid  the 
flow  of  blood  through  this  so-called  "  pumping "  action  on 
the  veins.  (2)  A  similar  pumping  action  on  the  veins  is  ex- 
erted by  alternate  flexions  (bendings)  and  extensions  at  any 
joint.  In  general,  flexions  force  the  blood  out  of  the  veins, 
while  extensions  allow  them  to  fill.  When  we  remember  how 
largely  most  of  our  usual  muscular  actions  consist  of  alter- 
nate flexions  and  extensions  of  joints  and  alternate  contrac- 
tions and  relaxations  of  muscles  (for  example,  in  walking 
and  running),  we  can  at  once  appreciate  how  greatly  mus- 
cular activities  must  aid  the  circulation.  When  to  the  effect 
of  these  we  add  the  suction  action  of  the  deepened  breathing 
movements,  the  effect  upon  the  circulation  becomes  very  great. 


FIG.  73.   The  pocket  valves  in  the  veins 

On  the  right  is  shown  the  external  appearance 
of  the  vein  at  the  valves  when  the  latter  are 
closed ;  on  the  left,  a  vein  slit  lengthwise  and 
opened ;  in  the  middle,  a  longitudinal  section 
of  a  vein 


150  THE  HUMAN  MECHANISM 

13.  Massage.    The  action  of  massage  is  only  another  illus- 
tration of  the  same  principle.    By  rubbing  the  legs  and  arms 
in  the  direction  of  the  heart,  the  blood  contained  in  their 
veins  is  forced  onward  and  the  circulation  aided,  precisely 
as  when  a  muscle  contracts  or  one  member  of  a  limb  is 
flexed  upon  another. 

14.  The  lymphatics.    Important  as  are  the  suction  action 
of  the  breathing  movements  and  the  pumping  action  of  con- 
tracting muscles  as  aids  to  the  circulation  of  the  blood,  they 
are  even  more  important  as  causes  of  the  flow  of  lymph  along 
the  greater  lymphatic  trunks  toward  the  heart.    Reference  to 
the  general  method  of  origin  of  lymphatics,  as  described  in 
Chapter  III,  will  show  that  the  lymph  in  the  lymph  spaces, 
unlike  the  blood  in  the  capillaries,  has  not  behind  it  a  high- 
pressure  reservoir ;  there  is  no  such  force  from  behind  to  send 
it  onward,  since  the  lymphatics  arise  blindly  in  the  tissues. 
What,   then,   makes  the   lymph  flow   along  the   lymphatics 
toward  the  heart? 

The  lymphatics  resemble  the  veins  in  structure,  having 
thin  walls  and  pocket  valves;  like  the  veins,  most  of  them 
originate  in  extrathoracic  organs  and  join  or  combine  to 
form  larger  trunks  as  they  proceed  toward  the  thorax.  All 
of  them  finally  unite  in  two  large  lymphatics  within  the 
thoracic  cavity,  and  these  open  into  the  great  veins  near  the 
heart.  (Figs.  30  and  70  should  be  consulted  in  this  con- 
nection.) It  is  at  once  clear  that  the  breathing  movements 
must  exert  on  the  lymph  within  these  thin-walled  vessels 
exactly  the  same  suction  action  as  they  exert  on  the  blood 
in  the  veins,  and  anything  which  increases  this  suction  action, 
such  as  the  deepened  breathing  movements  during  muscular 
activity,  must  necessarily  increase  the  flow  of  lymph  from 
every  organ  of  the  body.  On  the  other  hand,  a  pumping 
action  on  the  lymph  in  the  organs  results  from  all  rhythmic 
movements  of  parts  of  the  body  with  reference  to  one  another, 
since  each  change  of  position  carries  with  it  some  change  of 


THE  CIRCULATION  OF  THE  BLOOD  151 

external  pressure  on  lymphatics.  Familiar  examples  are  the 
movements  of  arms  and  legs  in  locomotion,  of  the  diaphragm 
in  breathing,  and  of  the  lungs  in  respiration. 

It  has  also  been  supposed  that  a  third  cause  of  the 
lymph  flow  is  the  passage  of  waves  of  constriction  (peri- 
stalsis, cf.  p.  125)  over  the  larger  lymphatics.  This,  however, 
probably  plays  only  a  minor  part. 

Finally,  in  the  formation  of  lymph  from  the  blood,  more 
water  generally  passes  from  the  capillaries  to  the  lymph 
spaces  than  from  the  lymph  spaces  into  the  capillaries. 
Under  these  circumstances,  at  least  at  certain  times,  the 
lymph  spaces  become  distended  and  a  certain  low  pressure 
obtains  in  them.  This  we  may  speak  of  as  the  "  active 
force  "  of  lymph  formation,  and  it  constitutes  a  fourth  factor 
in  causing  the  lymph  flow. 

We  have  already  pointed  out  the  importance  of  the  lymph 
flow  in  maintaining  the  lymph  currents  about  the  living 
cells ;  we  are  now  able  to  appreciate  the  importance  of  those 
agents  which  secure  this  flow.  As  enumerated  above,  they 
are  four  in  number:  (1)  suction  action  of  the  breathing 
movements ;  (2)  pumping  action  of  muscular  or  passive 
movements ;  (3)  active  force  of  lymph  formation ;  (4)  peri- 
staltic contractions  of  the  large  lymphatics. 

Of  these  the  fourth  is  at  least  doubtful  and  in  no  case  of 
great  importance ;  the  other  three  may  therefore  be  regarded 
as  the  chief  causes  of  the  lymph  flow,  and  of  these  the  first 
arid  second  are  brought  into  most  effective  action  by  mus- 
cular activity;  this  deepens  the  breathing  movements  and 
so  increases  their  suction  action  on  the  lymph,  while  the 
movements  of  the  body  exert  on  the  lymphatics  a  pumping 
action  which  is  largely  lacking  during  complete  inactivity. 
The  great  practical  importance  of  this  aspect  of  the  subject 
will  be  discussed  beyond  in  those  chapters  which  deal  with 
the  hygiene  of  muscular  activity  (Part  II). 


152  THE  HUMAN  MECHANISM 

O.  THE  ADJUSTMENT  OF  THE  CIRCULATION  TO  THE 
NEEDS  OF  EVERYDAY  LIFE 

The  total  quantity  of.  blood  in  the  body  (ten  to  fourteen 
pints)  is  riot  enough  to  furnish  a  working  supply  to  all 
organs  at  the  same  time ;  and  since,  in  general,  whenever 
an  organ  works  it  receives  more  blood,  and  when  it  is  at  rest 
it  receives  less,  our  daily  life  with  its  changes  of  activity 
among  the  organs  makes  necessary  frequent  adjustments  of 
the  circulation  to  the  needs  of  the  organs  at  various  times. 

Some  of  these  adjustments  are  matters  of  familiar  expe- 
rience. The  increased  flow  of  blood  to  the  skin  on  a  warm 
day  makes  the  veins  stand  out  and  the  face  red,  and  we  are 
conscious  of  the  more  rapid  heart  beat  during  muscular 
activity,  even  in  an  act  so  simple  as  running  upstairs.  Other 
adjustments  are  not  so  evident,  but  betray  themselves  by 
their  results,  as  happens  after  a  hearty  meal  when  the 
demand  of  the  digestive  organs  for  blood  lessens  the  supply 
to  the  brain  and  we  feel  disinclined  to  hard  mental  work. 
We  may  begin  our  study  of  these  adjustments  by  learning 
what  occurs  in  the  circulation  during  some  of  the  more 
common  activities  and  events  of  daily  life. 

15.  The  circulation  during  exposure  to  heat  and  cold.  When 
the  skin  is  exposed  to  cold  its  blood  supply  is  greatly  dimin- 
ished ;  the  veins  no  longer  stand  out  prominently  on  the  hand, 
and  if  a  small  area  of  skin  be  made  pale  by  pressing  upon  it 
(thus  driving  the  blood  out  of  its  capillaries),  the  pallor  passes 
off  very  slowly.  This  simple  experiment  shows  that  blood  is 
flowing  but  slowly  from  the  arterial  reservoir  into  the  skin. 
Conversely,  on  a  warm  day  the  veins  stand  out  prominently 
and  the  red  color  instantly  returns  upon  the  removal  of  pres- 
sure. These  variations  in  the  supply  to  the  skin  are  due,  as 
we  have  already  seen  (p.  147),  to  changes  in  the  diameter  of 
the  arteries  of  the  skin,  which  changes  serve,  like  the  spigot 
of  an  ordinary  water  faucet,  to  regulate  the  flow  of  liquid. 


THE  CIRCULATION  OF  THE  BLOOD  153 

The  changes  in  the  blood  flow  through  the  skin  are 
accompanied  by  corresponding  but  inverse  changes  in  the 
internal  organs.  On  a  cold  day  the  stomach  and  intestines, 
the  pancreas,  the  liver,  the  kidneys,  etc.  are  richly  supplied 
with  blood,  while  on  a  warm  day  their  blood  supply  is 
diminished.  In  the  former  case  the  blood  withheld  from  the 
skin  finds  its  way  into  the  internal  organs ;  in  the  latter  case 
the  skin  draws  upon  these  organs  for  its  needed  supply. 
The  circulation  in  the  internal  organs  compensates  for  that 
in  the  skin. 

16.  The  reason  for  compensatory  changes.  We  have  seen 
that  it  is  the  function  of  the  heart  to  keep  the  arterial  reser- 
voir adequately  distended  with  blood,  thus  supplying  a  steady 
driving  force  for  the  flow  of  blood  through  the  organs.  When 
the  small  arteries  of  the  skin  widen  on  a  warm  day,  blood 
escapes  more  rapidly  into  the  skin  from  the  arterial  reservoir. 
This  alone  would  diminish  the  amount  of  blood  in  the  reser- 
voir unless  the  heart  pumped  more  blood  or  unless  the  dila- 
tion or  widening  of  the  cutaneous  arterioles  were  compensated 
by  a  constriction  elsewhere,  so  that  the  total  drain  on  the 
reservoir  remained  the  same.  In  the  case  in  question  it  is 
the  latter  of  these  alternatives  which  is  adopted,  and  the 
reservoir  is  kept  filled  without  calling  on  the  heart  to  pump 
more  blood. 

Conversely,  on  a  cold  day  the  diminution  of  the  outflow 
into  the  skin  would  lead  to  a  backing  up  or  accumulation 
of  blood  in  the  great  arteries,  and  so  to  their  increased  and 
perhaps  undesirable  distention,  if  the  dilation  of  the  arte- 
rioles of  internal  organs  did  not  provide  an  outlet  for  the 
surplus  blood. 

Nowhere,  perhaps,  is  this  principle  of  compensatory  dila- 
tion or  constriction  of  arteries  in  one  region,  to  allow  for  the 
effect  of  the  opposite  change  in  some  other  region,  so  highly 
developed  or  so  fully  applied  as  in  the  reactions  of  the  body 
to  changes  in  external  temperature. 


154  THE  HUMAN  MECHANISM 

17.  The  circulation  during  muscular  activity.  During  mus 
cular  activity  the  arterioles  of  the  muscles  and  of  the  skin 
are  dilated,  the  former  in  order  to  supply  more  blood  to  the 
working  organ,  the  latter  to  aid  in  the  discharge  of  the  ex- 
cess of  heat  produced  by  the  contracting  muscles.  The  heavy 
drain  upon  the  arterial  reservoir  by  these  two  large  areas 
(among  the  largest  in  the  body)  is  compensated  to  some 
extent  by  the  constriction  of  the  arteries  of  the  digestive  and 
other  internal  organs.  This  alone,  however,  would  not  suffice 
to  keep  the  arterial  reservoir  filled ;  and  we  accordingly  find 
that  the  heart  beats  more  rapidly  and  more  powerfully, 
pumping  more  blood  into  the  aorta  in  a  given  time. 

It  is  very  important  to  remember  that  muscular  activity 
is  the  one  condition  of  life  which  materially  increases  the 
work  of  the  heart ;  at  other  times  the  greater  demand  of 
blood  for  the  working  organ  is  met  more  or  less  success- 
fully by  withdrawing  blood  from  a  resting  organ,  while  the 
supply  to  the  whole  arterial  system,  and  hence  the  work 
of  the  heart,  remains  approximately  unchanged.  During 
muscular  exercise,  and  then  only,  is  the  heart  called  upon 
for  decidedly  increased  work ;  and,  as  with  skeletal  muscles, 
its"  strength,  its  ability  to  meet  strain  and  emergencies  and 
to  withstand  fatigue,  depend  to  a  great  extent  upon  the 
training  given  it  in  this  way. 

Muscular  activity  also  influences  the  circulation  indirectly 
by  increasing  the  action  of  its  secondary  driving  forces  — 
the  suction  action  of  the  respiratory  movements  and  the 
pumping  action  of  the  contracting  muscles  on  the  veins. 
These  are  among  the  most  important  effects  of  this  agent 
upon  the  flow  of  blood,  but  they  are  too  complicated  for 
detailed  discussion  here. 

It  is  sometimes  stated  that  muscular  exercise  "  quickens  " 
the  circulation.  This  is  true  in  the  sense  that  the  heart 
pumps  more  blood  into  the  pulmonary  artery  and  the  aorta 
than  during  rest.  From  this  it  follows  that  during  exercise 


THE  CIRCULATION  OF  THE  BLOOD 


155 


more  blood  flows  through  the  lungs  and  that  blood  flows 
more  rapidly  out  of  the  arterial  reservoir,  but  it  does  not 
mean  that  blood  flows  more  rapidly  through  all  organs,  for 
the  digestive  and  other  internal  organs  at  such  times  actually 
receive  less  blood.  Indeed,  we  may  say  that  the  quickening 


FIG.  74.    Simple  apparatus  to  illustrate  the  relation  between  the  output  of 
the  heart,  the  peripheral  resistance,  and  the  general  arterial  pressure 

The  amount  delivered  by  the  faucet  represents  the  output  of  the  heart,  and  is  one 
factor  in  keeping  up  arterial  pressure ;  two  alternative  routes  of  outflow,  each 
capable  of  regulation,  represent  the  arterioles  to  different  organs.  Compensatory 
constrictions  and  dilations  and  other  hydraulic  conditions  described  in  the  text 
may  readily  be  imitated 

of  the  circulation  during  exercise  is  chiefly  confined  to  three 
important  organs  —  the  muscles,  the  skin,  and  the  lungs ;  in 
other  organs  the  change  is  relatively  slight,  as,  for  example,  in 
the  brain ;  while  in  still  others,  notably  those  of  the  digestive 
system  and  the  kidneys,  the  speed  is  diminished. 

18.  The  circulation  during  sleep.    An  adequate  blood  sup- 
ply is  necessary  to  the  full  activity  of  the  brain;  when  the 


156 


THE  HUMAN  MECHANISM 


circulation  in  this  organ  is  seriously  interfered  with,  imperfect 
mental  action  or  even  unconsciousness  is  a  result.  Thus 
when  all  the  arterioles  of  the  body  dilate,  or  the  heart  beat 
is  slowed  down,  in  consequence  of  some  sudden  "  shock,"  so 

that  pressure  in  the  arterial 
reservoir  falls  too  far,  the 
driving  force  for  the  flow 
of  blood  through  the  brain, 
as  well  as  through  other  or- 
gans, is  diminished,  and.  the 
person  loses  consciousness, 
or  faints.  Most  cases  of 
fainting  are  traceable  to 
one  or  the  other  of  these 
causes. 

The  most  familiar  and 
most  common  example  of 
unconsciousness,  however, 
is  that  of  sleep,  which  in 
so  many  respects  resembles 
fainting  as  to  suggest  that 
the  unconsciousness  in  both 
cases  is  due  to  the  same 
cause,  namely  a  lessened 
blood  supply  to  the  brain. 
Unquestionably,  the  amount 
of  blood  flowing  through 
the  brain  is  greatly  lessened 
during  sleep.  The  evidence 
for  this  statement  cannot 
be  given  here  in  full,  but  it 
is  known  that  where  accident  has  destroyed  a  part  of  the 
rigid  bone  of  the  skull,  and  the  wound  has  been  covered 
over  by  connective  tissue  and  skin,  the  scar  sinks  in  dur- 
ing sleep  —  indicating  less  blood  in  the  brain  —  and  returns 


FIG.  75.   Showing  the  relation  between 

general  arterial  tone  and  the  supply  of 

blood  to  the  brain 

In  A  the  arterioles  of  the  organs  m,  n,  s 
are  constricted,  raising  general  arterial 
pressure,  which  forces  a  large  amount  of 
blood  through  the  brain  b.  In  B  the  ar- 
terioles of  m,  n,  and  s  are  dilated,  general 
arterial  pressure  is  low,  and  less  blood  is 
forced  through  the  brain.  //,  heart 


THE  CIRCULATION  OF  THE  BLOOD  157 

to  the  level  of  the  general  surface  of  the  head  when  the 
subject  awakens. 

Upon  this  point  of  diminished  blood  supply  to  the  brain 
during  sleep  almost  all  physiologists  are  agreed;  there  is  also 
general  agreement  that  the  arm  and  the  leg  increase  in  vol- 
ume when  we  go  to  sleep,  and  this  is  thought  to  be  due  to 
a  dilation  of  the  arteries  of  the  skin.  It  is  very  significant, 
on  the  other  hand,  that  the  arm  shrinks  in  volume  when  the 
brain  is  active  in  mental  work,  and  especially  in  mental  work 
involving  the  personal  interest  or  mental  concentration  of  the 
subject  of  the  experiment. 

It  is  thought  by  some  that  other  vascular  areas  —  that  of 
the  abdominal  cavity,  for  example — behave  in  this  respect  in 
the  same  way  as  the  skin,  but  on  this  point  the  evidence  is 
not  conclusive.  It  is,  indeed,  not  improbable  that  these  other 
vascular  areas  play  some  part  in  the  regulation  of  the  flow 
to  the  brain,  but  it  is  not  likely  that  they  stand  in  the  same 
intimate  relation  to  it  as  does  the  skin. 

The  fact  is  clear,  however,  that  a  close  relation  exists 
between  cutaneous  circulation  and  the  maintenance  of  proper 
vascular  conditions  in  the  brain.  Mental  work,  for  example, 
is  more  difficult  for  most  people  in  very  warm  weather 
because  at  that  time  the  cutaneous  arterioles  are  widely 
dilated ;  and,  on  the  other  hand,  it  is  easy  to  understand 
why  the  constriction  of  the  vessels  of  the  skin  by  cold  makes 
it  difficult  to  go  to  sleep  without  sufficient  bedclothing. 

19.  The  circulation  during  the  digestion  of  a  meal.  After 
eating  a  meal  more  blood  is  needed  in  the  secreting  diges- 
tive glands  (especially  the  stomach  and  pancreas)  and  also 
in  the  intestinal  organs  of  absorption,  the  villi.  This  need 
is  greatest  during  the  first  hour  or  two,  when  there  is  the 
largest  amount  of  food  to  be  worked  upon.  We  find,  accord- 
ingly, that  the  arteries  of  these  organs  then  dilate  to  such 
an  extent  that  the  mucous  membrane  of  the  stomach  and 
intestine,  which  is  pale  pink  while  those  organs  are  at  rest, 


158  THE  HUMAN  MECHANISM 

now  becomes  very  red  on  account  of  the  large  amount  of 
blood  flowing  through  them. 

There  is  probably  some  compensation  for  this  in  other 
organs,  but  it  is  an  imperfect  compensation.  The  drowsiness 
which  is  apt  to  come  on  after  a  hearty  meal  is  probably  an 
indication  that  these  compensations  are  not  complete  and 
that  owing  to  the  fall  of  arterial  pressure  the  brain  is  not 
receiving  its  normal  blood  supply. 

20.  Some  practical  applications.  We  may  pause  here  to 
consider  some  important  practical  applications  of  these  facts. 
While  the  most  active  secretion  is  in  progress  nothing  should 
be  done  which  will  take  blood  away  in  large  quantities  from 
the  stomach.  Muscular  exercise,  for  example,  then  as  always, 
dilates  the  arterioles  of  the  muscles  and  skin  and  constricts 
those  of  the  digestive  organs ;  this  is  obviously  an  unfavorable 
vascular  condition  for  the  act  of  secretion.  If  the  meal  be  a 
light  one,  so  that  comparatively  little  of  the  digestive  juices 
are  required,  no  harm  may  be  done  by  taking  exercise  after 
a  meal ;  but  where  the  meal  is  heavier  it  is  almost  always 
unwise,  especially  in  warm  weather.  Similar  considerations, 
which  are  likewise  in  full  accord  with  experience,  indicate 
that  it  is  unwise  to  eat  as  large  meals  in  very  warm  weather 
as  in  cooler  weather.  The  larger  the  meal,  the  greater  the 
amount  of  gastric  juice  required  to  start  its  digestion;  but 
in  warm  weather  the  arteries  of  the  stomach  and  intestine 
tend  to  be  constricted  (see  p.  152),  so  that  it  is  difficult  to 
secure  an  adequate  blood  flow  through  these  organs,  and 
their  efficiency  is  to  this  extent  impaired. 

It  is  sometimes  stated  that  mental  work  immediately  after 
meals  causes  indigestion  by  taking  blood  away  from  the 
digestive  organs  and  sending  it  to  the  brain.  It  is  very 
doubtful,  however,  whether  the  increased  blood  flow  to  the 
brain  is  secured  largely  at  the  expense  of  that  to  the  diges- 
tive organs.  While  instances  might  be  cited  of  indigestion 
among  people  who  do  mental  work  upon  a  "full  stomach," 


THE  CIRCULATION  OF  THE  BLOOD  159 

it  must  be  remembered  that  these  are  usually  people  who 
fail  to  take  proper  exercise  or  sufficient  sleep  and  rest;  the 
indigestion  from  which  they  too  frequently  suffer  is  more 
often  attributable  to  these  causes  than  to  the  fact  that  the 
digestive  organs  are  deprived  of  their  proper  blood  supply. 

21.  The  mechanism  of  the  regulation  of  the  flow  of  blood. 
Having  thus  considered  exactly  what  takes  place  in  the  circu- 
lation during  some  of  the  more  important  events  of  daily  life, 
we  may  next  inquire  briefly  into  the  physiological  mechanism 
by  which  these  adjustments  are  secured.    Its  most  important 
features  are  the  regulation  of  the  inflow  from  the  heart  into 
the    arterial    reservoir    and    the    regulation    of    the    outflow 
through  the  arterioles  and  capillaries  of  the  organs.    These 
two  must  be  adjusted  to  each  other  in  order  that  the  reser- 
voir may  remain  full  and  thus  the  driving  force  for  the  flow 
through  the  organs  be  maintained.    We  shall  go  into  the  de- 
tails of  this  very  beautiful  but  complicated  mechanism  only 
far  enough  to  enable  the  student  to  appreciate  certain  prin- 
ciples  of  fundamental   importance  in  the  practical  conduct 
of  life. 

22.  The  regulation  of  the  pumping  action  of  the  heart.    The 
amount  of  blood  which  the  heart  pumps  varies  considerably 
from  time  to  time.    At  times  it  may  be  as  low  as  three 
quarts  a  minute  and  at  other  times  as  high  as  twelve  quarts, 
the  quantity  being  largely  determined  by  the  drain  made  at 
the  time  upon  the  arterial  reservoir.    It  will  be  seen  at  once 
that  this  involves  a  wide  range  of  adjustment. 

The  beat  of  the  heart  is  primarily  due  to  events  which 
take  place  within  the  heart  itself.  We  have  seen  that  this 
beat  is  a  muscular  contraction.  But  the  cardiac  muscle  dif- 
fers from  the  skeletal  muscle  in  that  it  does  not  require  an 
impulse  from  the  central  nervous  system  to  throw  it  into 
activity.  When  the  heart  is  cut  off  from  connection  with 
the  rest  of  the  body,  it  continues  to  beat  for  a  time  and,  if 
supplied  with  warm  blood,  it  may  be  kept  beating  for  hours. 


160  THE  HUMAN  MECHANISM 

We  express  this  by  saying  that  the  heart  beat  is  automatic, 
by  which  we  mean  that  the  heart  contains  within  itself  a 
complete  mechanism  for  doing  its  own  work. 

23.  The  augmentor  and  the  inhibitory  nerves  of  the  heart. 
Nevertheless   the  heart   receives   from    the   central   nervous 
system  two  pairs  of  nerves  which  are  able  to  influence  the 
rate  and  the  force  of  the  automatic  beats.    One  pair  of  these 
nerves  carries  from  the  spinal  cord  to   the  heart  impulses 
which  stimulate   that  organ   to  beat  more  rapidly  or  more 
forcibly,  or  both.    Hence  these  are  known  as  the  augmentor, 
or  accelerator,  nerves. 

The  fibers  of  the  other  pair  of  nerves  produce  exactly 
the  opposite  effect.  Running  from  the  lower  part  of  the 
brain,  they  carry  to  the  heart  impulses  which  slow  the  beat 
or  lessen  its  force,  or  they  may  produce  both  effects  at  the 
same  time.  They  act,  as  it  were,  like  a  brake  on  a  wheel, 
checking  the  activity  of  the  automatic  beat.  These  fibers 
are  known  as  inhibitory  fibers,  and  their  action  is  a  case  of 
inhibition. 

24.  Inhibition.    In  the  examples  of  nervous  action  which 
we  have  thus  far  studied,  the  nervous  impulse  has  uniformly 
thrown  some  cell  into  activity.    The  stimulation  of  muscle 
fibers  to  contract,  of  gland  cells  to  secrete,  and  of  nerve  cells 
in  the  execution  of  reflexes  will  be  readily  recalled.    To  this 
same  class  of  nervous  actions  must  now  be  added  that  of 
the  augmentor  nerves  of  the  heart,  for  they  excite  the  heart 
to  greater  activity. 

In  the  inhibitory  nerves,  on  the  other  hand,  the  nervous 
impulse  produces  exactly  the  opposite  result.  Instead  of  set- 
ting organs  to  work  or  stimulating  them  to  more  vigorous 
action,  they  diminish  activity  and  in  extreme  cases  check  or 
stop  it  altogether.  In  our  subsequent  studies  we  shall  meet 
with  many  examples  of  this  effect;  but  we  may  say  at  once 
that  inhibition  is  as  characteristic  and  as  important  a  feature 
of  the  nervous  system  as  is  excitation  (see  p.  281). 


THE  CIRCULATION  OF  THE  BLOOD  161 

25.  The  regulation  of  the  outflow  from  the  arterial  reservoir; 
arterial  tone.  Wound  around  the  walls  of  the  arterial  tubes, 
especially  the  smaller  arteries  (arterioles)  which  deliver  blood 
from  the  arterial  reservoir  to  the  organs,  are  peculiar  muscle 
fibers.  Their  contraction  diminishes  the  size  and  bore  of  the 
tube,  and,  when  they  relax,  the  tube  and  its  lumen  become 
wider.  As  a  usual  thing  these  smaller  arteries  are  kept  some- 
where midway  between  extreme  constriction  and  extreme 
dilation.  On  a  day  of  moderate  temperature,  for  example, 
the  arterioles  of  the  skin  are  moderately  narrowed  by  this 
action  of  their  muscle  fibers.  During  colder  weather  these 
fibers  contract  more  than  usual  and  so  lessen  the  size  of  the 
tube,  while  during  warm  weather  they  relax  somewhat  and 
widen  it;  but  ordinarily  they  are  never  contracted  to  their 
utmost  nor  are  they  often  completely  relaxed. 

This  condition  of  sustained  activity  of  the  arterial  muscles 
is  known  as  arterial  tone,  and  in  general  any  sustained  activity 
of  a  living  cell  is  spoken  of  as  tonic  activity,  or  tone.  Since, 
as  we  have  seen,  the  total  quantity  of  blood  in  the  body  is 
not  enough  to  fill  completely  and  distend  all  the  blood  ves- 
sels when  they  are  widened  to  their  utmost,  it  follows  that 
the  maintenance  of  arterial  tone  is  essential  to  that  overfilling 
of  the  great  arteries  which  supplies  the  driving  force  for  the 
flow  of  blood  through  the  organs.  If  every  arteriole  were  to 
lose  its  tone,  blood  would  flow  out  of  the  reservoir  more 
rapidly  than  the  heart  could  possibly  pump  it  in ;  we  should 
have  somewhat  the  same  condition  of  affairs  as  if,  in  our  arti- 
ficial model  (p.  144),  the  small  nozzle  which  affords  resist- 
ance to  the  outflow  were  removed.  Arterial  pressure  would 
fall  and,  the  driving  force  being  thus  removed,  the  blood 
would  remain  at  rest  in  the  capillaries  and  veins  of  the 
organs;  the  circulation  would  cease  because  blood  would 
not  return  to  the  heart  to  be  pumped.  The  maintenance 
of  arterial  tone  is  consequently  no  less  essential  to  the 
circulation  than  is  the  beat  of  the  heart  itself. 


162  THE  HUMAN  MECHANISM 

Two  means  are  known  by  which  the  contraction  of  the 
circular  muscle  fibers  of  the  arterioles  is  regulated:  first, 
impulses  from  the  central  nervous  system  over  the  vasomotor 
nerves;  and,  second,  direct  excitation  of  the  arterioles  by 
hormones  in  the  circulating  blood.  The  vasomotor  nerves 
are  of  two  kinds,  the  vasoconstrictors  and  the  vasodilators; 
the  best-known  hormone  acting  on  the  arterioles  is  adre- 
naline, the  action  of  which  has  already  been  referred  to  in 
Chapters  VI  and  VII. 

26.  Vasoconstrictor  nerves.    The  muscle  fibers  of  the  arter- 
ies receive  nerves  which  stimulate  them  to  contract,  for  if 
these   nerves  are   cut,  the  arteries  lose  their  tone    (dilate). 
We   conclude,  therefore,  that  the   ordinary  maintenance  of 
arterial  tone  is,  in  part  at  least,  a  function  of  the  nervous 
system.    The  muscle  fibers  of  the  arteries,  in  other  words, 
remain  in  tonic  activity  because  the  neurones  which  supply 
them  with  nerve  fibers  are   in  tonic  activity;    and  we  can 
understand  how  general  arterial  tone  may  be  increased  or 
decreased  by  the  condition  of  the  central  nervous  system,  by 
reflexes,  by  the  nervous  "  shock  "  of  surgical  operations,  etc. 

Neurones  which  maintain  the  proper  amount  of  arterial 
tone  are  known  as  vasoconstrictor  neurones.  They  obviously  do 
for  the  muscles  of  the  arteries  what  the  motor  nerves  do  for 
the  skeletal  muscles,  and  the  augmentors  do  for  the  heart. 

27.  Vasodilator  nerves.    Many  arteries,  however,  receive  a 
second  set  of  nerves,  which  have  exactly  the  opposite  func- 
tion, that  is,  to  make  their  muscle  fibers  relax  and  so  lead  to 
a  widening  or  dilation  of  the  artery.    These  nerves  do  for  the 
tonic  contraction  of  the  arteries  what  the  inhibitory  nerves  of 
the  heart  do  for  the  heart  beat  —  they  diminish  or  abolish 
an  existing  activity  and  thus  give  us  our  second  example 
of  inhibitory  nerves.    They  are  known  as  the  vasodilators. 

The  vasodilators  are  not  regularly  in  tonic  activity  like 
the  vasoconstrictors.  They  are  called  into  action,  reflexly  or 
otherwise,  when  it  is  necessary  that  an  organ  receive  more 


THE  CIRCULATION  OF  THE  BLOOD  163 

blood  than  usual;  at  other  times  the  vasoconstrictors  are 
free  to  exert  their  tonic  stimulation  and  so  regulate  the  flow 
of  blood  to  the  organs. 

28.  The  regulation  of  arterial  tone  by  hormones ;  adrenaline. 
This  has   already  been   described    on  page  65.     It  will    be 
recalled  that  the  presence   of  adrenaline  in  the  circulating 
blood  directly  excites  the  arterioles  to  constrict;    that  this 
action  on  the  arterioles  is  greater  in  some  regions  (for  ex- 
ample, the  abdominal  organs)  than  in  others  (for  example, 
the  skeletal  muscles  and  skin) ;  that  the  rate  and  force  of 
the  heart  beat  are  influenced;    that  the  adrenal  glands  are 
excited  to  secrete  by  nervous  impulses  which  are  dispatched 
from  the  central  nervous  system  during  states  of  emotional 
excitement  (fear  and  anger)  and,  we  may  now  add,  when- 
ever the  blood  is  deficient  in  oxygen.    There  are  also  reasons 
for  thinking  that  the  internal  secretion  of  the  pituitary  body 
(p.  67)  may  likewise  play  some  role  in  regulating  arterial 
tone  and  possibly  in  the  distribution  of  the  blood  among  the 
organs.    This  is  a  new  field  of  physiology  and  the  present 
state  of  our   knowledge    justifies  only   this  brief   reference 
to  it.    Enough  is  known,  however,  to  show  that  hormones 
cooperate  with  the  vasomotor  nerves  in  regulating  the  flow 
of  blood  to  the  organs. 

29.  Importance  of  the  vascular  adjustments  in  daily  life. 
It  is  not  possible  within  the  limits  of  the  present  work  to 
enter  further  into  the  mode  of  action  of  these  factors  of  vas- 
cular coordination.    Our  main  purpose  is  to  show  the  student 
that  proper  coordination  is  as  important  in  adapting  the  work 
of  the  heart  and  blood  vessels  to  the  hourly  needs  of  daily 
life  as  it  is  in  producing  purposeful  movements  of  the  skeletal 
muscles.    Every   change   of   occupation   and   activity,   every 
change  of  surrounding  conditions  of  temperature,  moisture, 
wind,  etc.,  necessitates  some  special  adjustment  of  the  vas- 
cular system;    and  this  adjustment  is  dependent  upon  the 
same   sort  of  coordinating   action   which   we    have   already 


164  THE  HUMAN  MECHANISM 

compared  with  the  operations  of  a  large  army.  In  spite  of 
the  fact  that  we  are  for  the  most  part  uriconscious  of  it,  it  is 
none  the  less  a  part  of  our  daily  life;  and  the  fatigue  induced 
within  these  vasomotor  and  hormone  mechanisms  by  their 
continued  activity  probably  contributes  a  large  share  to  that 
general  bodily  fatigue  which  leads  us  to  seek  recuperation 
in  rest  and  sleep. 

The  apparatus,  the  operation,  and  the  regulation  of  the 
flow  of  blood  and  lymph  afford  an  excellent  illustration  of 
the  fact  that  the  human  body,  at  least  in  this  particular,  is 
a  complex  machine.  But  while  we  of  to-day  look  upon  it 
with  somewhat  less  of  awe  than  did  our  ancestors,  and 
while  there  is  for  us  less  of  mystery  and  more  of  mechanism 
in  it,  we  gain,  on  the  other  hand,  a  wholly  new  revelation 
of  its  intricacy  and  a  fresh  sense  of  its  marvelous  delicacy, 
beauty,  and  perfection  of  adjustment.  The  mere  fact  that 
everyone  of  us  carries  in  his  bosom  a  powerful  double 
force-pump  of  remarkable  design,  original  construction,  and 
extraordinary  power,  capable  in  many  instances  of  success- 
ful and  unremitting  service  for  more  than  three  quarters  of 
a  century,  should  be,  in  itself  alone,  enough  to  excite  admi- 
ration and  respect  for  the  entire  mechanism  of  which  it  is 
only  one  part  and  to  awaken  within  us  a  desire  to  use  that 
mechanism  "  as  not  abusing  it." 


CHAPTER  X 
BESPIEATION 

1.  The  fundamental  act  of  respiration.    We  have  found  in 
studying  the  chemical  changes  which  underlie  cellular  activity 
(Chap.  IV)  that  muscle  fibers  and  gland  cells  and,  we  may 
now  add,  nerve  cells  take  in  oxygen  and  give  out  carbon 
dioxide.    This  cell  breathing  is  the  essential  act  of  respiration, 
for  respiration  is  only  another  name  for  the  oxidative  proc- 
esses of  the  living  body.    Respiration  of  this  kind  (and  of 
this  kind  only)  is  universal  among  living  things.    The  one- 
celled  animal,  for  example,  takes  its  oxygen  directly  from 
the  free  oxygen  of  the  water  in  which  it  lives,  and  discharges 
its  carbon  dioxide  into  the  same  surrounding  medium.   Every 
one  of  the  thousands  of  cells  of  which  the  human  body  is 
composed  repeats  this  same  process,  taking  its  oxygen  from 
and    discharging    its    carbon    dioxide    into    its    surrounding 
medium  —  in    this    case    the    lymph.     The    breathing   move- 
ments, which  renew  the  air  in  the  lungs,  and  the  circulation 
of  blood,  which  affords  the  channel  of  communication  between 
the  Inngs  and  the  tissues,  are  merely  accessory  mechanisms 
rendered  necessary  by  the  distance  of  the  cells  and  the  lymph 
from  the  surface  of  the  body.    Their  principal  function  is  to 
keep  the  lymph  supplied  with  oxygen  and  to  remove  from  it 
the  carbon  dioxide.    In  other  words,  breathing,  though  minis- 
tering to  respiration,  is  not  respiration  itself. 

2.  The  quantity  of  oxygen  and  of  carbon  dioxide  in  the 
lymph  surrounding  the  cells  of  the  body.    The  cell  is  the  true 
seat  of  oxidation.    Within  its  imperfectly  understood  mecha- 
nism are  found  the  conditions  which  lead  to  the  union  of 

165 


166  THE  HUMAN  MECHANISM 

oxygen  with  the  proteins,  the  carbohydrates,  and  the  fats 
of  the  food. 

The  cell  draws  oxygen  from  the  surrounding  lymph  very 
much  as  a  burning  match  draws  oxygen  from  the  surround- 
ing air.  Consequently  the  amount  of  oxygen  dissolved  in 
the  lymph  is  generally  comparatively  small  and  would  be 
removed  altogether  were  it  not  constantly  renewed  from 
the  blood. 

For  similar  reasons  the  lymph  must  be  relatively  rich  in 
carbon  dioxide,  since  it  is  this  fluid  which  directly  receives 
the  gas  (in  solution)  from  its  source  of  manufacture,  the 
working  cell.1 

3.  The  quantity  of  oxygen  and  of  carbon  dioxide  in  arterial 
blood.  It  is  through  the  lungs  that  the  body  as  a  whole 
receives  its  oxygen  and  discharges  its  excess  of  carbon 
dioxide.  Consequently  arterial  blood  contains  more  oxygen 
and  less  carbon  dioxide  than  venous  blood.  The  actual 
figures  are  as  follows: 

OXYGEN        CARBON  DIOXIDE    NITROGEN 

100  cc.  of  arterial  blood  contain        20  cc.  38  cc.  1-2  cc. 

100  cc.  of  venous  blood  contain     8-12  cc.  45-50  cc.  1-2  cc. 

These  figures  apply  to  the  whole  blood,  that  is,  to  plasma 
and  corpuscles;  but  what  is  true  of  the  whole  blood  is  true 
in  a  general  way  also  of  the  circulating  plasma,  which  con- 
sequently enters  the  capillaries2  relatively  rich  in  oxygen 
and  poor  in  carbon  dioxide,  thus  presenting  exactly  the 
reverse  composition,  in  respect  to  these  gases,  of  that  found 
in  the  lymph  surrounding  the  living  cells. 

1  The  gases  oxygen  and  carbon  dioxide  are,  of  course,  dissolved  in  the 
liquid  lymph  and  blood  plasma.    A  liquid  exposed  to  a  gas  absorbs  or  dis- 
solves the  gas.    Thus  100  cc.  of  water  when  exposed  to  atmospheric  air  at 
0°C.  dissolves  4  cc.  of  oxygen  and  2  cc.  of  nitrogen. 

2  The  total  time  consumed  by  the  blood  in  passing  from  the  capillaries  of 
the  lungs  through  the  heart  to  those  of  the  rest  of  the  body  seldom  exceeds 
five  or  six  seconds.    Hence  the  amount  of  the  gases  in  the  blood  entering 
the  capillaries,  for  example,  of  a  muscle  is  practically  the  same  as  in  the 
blood  leaving  the  lungs. 


RESPIRATION 


167 


4.  The  exchange  of  oxygen  and  carbon  dioxide  between  the 
lymph  and  the  blood  plasma.    In  the  capillary  regions  of  all 
parts  of  the  body  except  the  lungs  we  have  two  fluids,  the 
lymph  and  the  blood  plasma,  containing  very  different  amounts 
of  oxygen  and  carbon  dioxide  and  separated  from  each  other 
by  the  exceedingly  thin  membrane  of  the  capillary  wall.  Under 
such  conditions  both  gases  will  tend  to  equalize,  and  each  gas 
will  pass  through  the  membrane  from  that  liquid  in  which  it 
is  more  abundant  to  that 

in  which  it  is  less  abun- 
dant; that  is  to  say,  the 
oxygen  will  pass  from  the 
blood  plasma  in  which  it 
abounds  to  the  lymph  in 
which  it  is  scarce ;  and 
the  carbon  dioxide,  in  the 
other  direction,  from  the 
lymph  to  the  blood  plasma 
(see  Fig.  76).  Hence  the 
blood  enters  the  veins 
richer  in  carbon  dioxide 
and  poorer  in  oxygen  than 
it  left  the  arteries. 

5.  The  red  corpuscle  as  a  carrier  of  oxygen.    The  blood 
plasma  under  the  conditions  of  temperature  and  pressure  to 
which  it  is  exposed  can  hold  only  a  small  amount  of  oxygen, 
too  little  to  meet  satisfactorily  the  demands  of  the  resting 
tissues  and  utterly  inadequate  for  the  much  greater  needs 
of  the  working  tissues.    This  difficulty  is  met  and  the  oxygen- 
carrying  capacity  of  the  blood  vastly  increased  by  the  peculiar 
properties  of  the  coloring  matter,  or  pigment,  of  the  red  cor- 
puscles.   This  substance,  known  as  hemoglobin,  readily  forms 
with  oxygen  a  compound  (oxyhemoglobiri)  whenever  the  amount 
of  oxygen  is  high  in  the  medium  surrounding  it ;  if,  however, 
much  oxygen  is  removed  from  its  surrounding  medium,  the 


FIG.  76.    The  exchange  of  oxygen  and 

carbon  dioxide  between  the  blood  and 

the  lymph  in  the  tissues 


x 


168  THE  HUMAN  MECHANISM 

oxyhemoglobin  breaks  up  or  dissociates  into  hemoglobin  and 
free  oxygen.  Applying  this  to  the  conditions  in  the  capil- 
laries, we  find  that  100  cc.  of  arterial  blood  contain  less  than 
1  cc.  of  free  oxygen  in  the  plasma,  but  about  19  cc.  of  oxy- 
gen combined  in  the  oxyhemoglobin  of  the  red  corpuscles. 
When  the  blood  enters  the  capillaries  of  living  tissues,  oxy- 
gen passes,  as  we  have  seen,  from  the  plasma  into  the  lymph, 
so  that  the  oxygen  content  of  the  plasma  is  reduced.  When 
this  reduction  goes  below  a  point  which  is  quickly  reached, 
dissociation  of  the  oxyhemoglobin  occurs,  and  the  oxygen 
thus  set  free  in  the  plasma  is  drawn  away  by  the  lymph, 
from  which  it  is  in  turn  drawn  by  the  cell,  the  real  seat 
of  oxidation. 

The  amount  of  oxygen  given  above  (20  cc.  to  100  cc.  of 
blood)  is  all  that  the  blood  can  hold  under  the  usual  con- 
ditions of  atmospheric  pressure  and  at  the  temperature  of  the 
body.  Moreover,  the  oxygen  content  of  the  blood  leaving 
the  lungs  (arterial  blood)  is  usually  kept  remarkably  con- 
stant by  the  accurate  adjustment  of  the  breathing  movements 
to  the  needs  of  the  body.  Neither  by  deeper  nor  by  more 
rapid  breathing  is  it  possible  to  increase  appreciably  the 
amount  of  oxygen  absorbed  by  the  same  volume  of  blood 
flowing  through  the  lungs.  Only  by  increasing  the  quantity 
of  blood  pumped  through  the  lungs  can  we  increase  the 
amount  of  oxygen  carried  to  the  organs  and  tissues;  and, 
for  the  same  reason,  only  by  increasing  the  quantity  of 
blood  flowing  through  an  organ  can  we  increase  the  oxygen 
supplied  to  that  organ. 

6.  The  consumption  of  oxygen  in  the  tissues.  The  quantity 
of  material  oxidized  in  the  cells  of  the  body  depends  chiefly, 
indeed  under  ordinary  conditions  of  life  it  depends  entirely, 
on  the  amount  of  work  these  cells  are  doing.  To  put  the 
matter  in  another  way,  the  cells  always  contain  a  certain 
quantity  of  oxidizable  material  formed  by  the  chemical 
changes  going  on  within  them;  during  work,  or  activity, 


EESPIRATION  169 

there  is  a  marked  increase  of  oxidizable  material  (possibly 
the  result  of  the  cleavages  described  in  Chapter  IV),  and 
for  this  reason  there  is  a  corresponding  increase  of  oxidation 
in  the  cell.  It  follows  that,  in  general,  cell  oxidation  can 
be  increased  only  by  increasing  cell  work ;  it  cannot  be  in- 
creased by  the  mere  act  of  deep  breathing.  "  We  may  lead 
a  horse  to  water  or  fetch  water  to  a  horse,  but  we  cannot 
make  him  drink."  The  assertion,  too  frequently  heard,  that 
some  special  form  of  breathing  movement  leads  to  more 
efficient  oxidation  of  wastes  throughout 
the  body  betrays  lamentable  ignorance  of 
this  fundamental  fact  of  physiology.  This, 
however,  is  not  denying  that  one  type  of 
breathing  movement  may  still  be  prefer- 
able to  another,  nor  affirming  that  deep- 
ened breathing  may  not  sometimes  be 
desirable.  Breathing  movements  accom-  FlG  77  Two  adja_ 
plish  other  things  than  oxygenation  of  the  cent  alveoli  of  the 
blood,  and  we  may  now  proceed  to  study  lun& 

their   physiology.    "  Showing  the  air  cells 

7.  Structure  of  the  lungs.  In  Chapter  II  the  anatomical 
relations  of  the  air  passages  (trachea,  bronchi,  etc.)  and  lungs 
have  been  described.  The  student  at  this  point  should  con- 
sult especially  Fig.  5  (p.  13)  in  order  to  obtain  a  clear  idea 
of  the  structure  of  the  lungs.  The  bronchi  which  enter  the 
lungs  branch,  much  as  the  ducts  of  a  gland,  and  their  ulti- 
mate branches  end  in  the  alveoli,  which,  like  those  of  a  gland, 
consist  of  a  single  layer  of  cells,  but  in  this  case  of  very  thin, 
flattened  cells.  Fig.  77  shows  two  of  these  alveoli  dissected, 
and  Fig.  78  a  section  taken  lengthwise  through  the  same. 
Connective  tissue  binds  together  the  alveoli  and  bronchial 
tubes,  thus  forming  the  lobes  of  the  lungs.  In  this  connective 
tissue — and  hence  between  the  alveoli — are  the  larger  blood 
vessels,  branches  of  the  pulmonary  artery  and  pulmonary 
veins.  The  arterioles  supply  an  exceedingly  close  network 


1TO 


THE  HUMAN  MECHANISM 


of  capillaries  (Fig.  159),  which  are  in  direct  contact  with  the 
lining  cells  of  the  alveolus,  so  that  the  blood  in  these  capil- 
laries is  separated  from  the  air  in  the  alveolus  only  by  the 
thin  capillary  wall  and  the  equally  thin  layer  of  flattened 
alveolar  cells.  Under  these  circumstances  the  exchange  of 
oxygen  and  carbon  dioxide  takes  place  readily  between  the 

air  in  the  lungs 
and  the  blood 
in  the  capillaries. 
Finally,  the  ab- 
sorbing surface 
of  the  alveolar 
wall  is  greatly 
increased  by  be- 
ing arranged  in 
the  form  of  pits, 
or  air  cells?-  as 
shown  in  Figs.  5, 
77,  78,  and  159. 
8.  Purpose  of 
breathing  move- 
ments. As  the 
blood  is  con- 
stantly giving  up 
carbon  dioxide  to, 
and  taking  oxy- 
gen from,  the  air 
of  the  lungs,  this  air  would  soon  cease  to  be  of  use  in 
purifying  the  blood  were  it  not  for  the  breathing  move- 
ments, whose  function  is  to  replace  the  vitiated  air  within  the 
lung  with  pure  air  from  without.  Breathing  is,  accordingly, 
an  act  of  ventilation  of  the  lungs,  and  it  is  the  stoppage  of 
this  ventilation  which  produces  suffocation,  or  asphyxia. 

1  The  word  "cell"  is  here  used  to  represent  a  hollow  space  and  not 
with  its  usual  histological  meaning. 


FIG.  78.    Diagram  of  a  longitudinal  section  of  two 
alveoli  with  their  common  bronchiole,  and  show- 
ing, in  black,  the  larger  blood  vessels  in  the  con- 
nective tissue 

The  capillary  network  belonging  to  these  vessels  is 
shown  in  Fig.  159 


RESPIRATION 


171 


9.  Mechanics  of  the  breathing  movements.  A  knowledge  of 
the  mechanism  of  the  breathing  movements  is  of  much  practi- 
cal importance,  especially  in  hygiene,  and  may  be  understood 
without  great  difficulty  by  the  study  of  the  model  shown  in 
Fig.  79.  The  trachea  and  the  bronchi  are  represented  by  the 
glass  tube,  and  the  lungs  by  an  elastic  bag,  Z,  at  the  end  of 
the  tube.  The  lungs  lie  in  the  large  air-tight  thorax,  which  in- 
closes the  pleural,  or  thoracic,  cavity  (p.  10).  This  thoracic  wall 
is  represented  in  the  model  by  a  glass  bell  jar  closed  beneath 
by  a  sheet  of  thick  rubber,  D.  The  cavity 
of  the  bell  jar  represents  the  pleural 
cavity,  and  the  rubber  represents  the  dia- 
phragm (see  Fig.  154).  The  condition  of 
the  lung  in  the  pleural  cavity  may  be 
still  further  imitated  in  the  model  by  fas- 
tening the  .inflated  rubber  bag  tightly  into 
the  jar.1  The  rubber  bag  remains  moder- 
ately inflated  within  the  air-tight  cavity 
of  the  bell  jar.  In  the  body  the  distended 
lungs  virtually  fill  those  portions  of  the 
thoracic  cavity  not  occupied  by  the  heart, 
great  blood  vessels,  and  other  organs.2 

Now  enlarge  the  "thoracic  cavity"  of  the  model  by  pulling 
downwards  on  the  sheet  of  rubber  which  represents  the  dia- 
phragm. The  "  lungs  "  within  will  expand  while  air  is  sucked 
through  the  glass  "  trachea  "  and  mixes  with  that  in  the  model 
"  lungs."  When  the  pull  is  released,  the  "  diaphragm  "  rises, 
thus  diminishing  the  size  of  the  "  thorax  "  and  so  forcing  air 
out  of  the  "  lungs."  In  this  way  the  mechanism  of  the  venti- 
lation of  the  lungs  may  be  imitated  in  essential  particulars. 

1  Loosen  the  rubber  stopper  and,  while  the  neck  of  the  bell  jar  is  open, 
inflate  the  rubber  bag  through  the  tube ;  while  the  bag  is  thus  inflated, 
push  the  rubber  stopper  down  into  the  neck  of  the  bottle. 

2  The  student  is  again  warned  against  supposing  that  the  pleural  cavity 
is  a  large  space  filled  with  air ;  in  this  respect  the  model  is  misleading,  since 
the  lungs  and  other  organs  completely  fill  the  thoracic  cavity. 


FIG.  79.  Model  of  the 
action  of  the  thoracic 
walls  and  lungs  in  res- 
piration (see  sect.  9) 


172  THE  HUMAN  MECHANISM 

In  life  the  pleural  cavity  is  enlarged  during  inspiration  by 
the  contraction  of  the  diaphragm  and  the  elevation  of  the 
ribs.  Both  of  these  are  movements  effected  by  the  action  of 
skeletal  muscles.  The  understanding  of  the  elevation  of  the 
ribs  need  give  no  difficulty ;  muscles,  some  of  which  are 
shown  in  Fig.  12,  pull  upwards  on  the  ribs;  and  the  attach- 
ment of  the  ribs  to  the  vertebral  column  and  the  breastbone 
(sternum)  is  such  that  when  they  are  raised  the  diameter  of 
the  thorax  is  increased  dorsoventrally  and  from  side  to  side. 
The  diaphragm,  on  the  other  hand,  is  a  kind  of  circular  .mus- 
cle with  a  central  fibrous  or  tendinous  portion  from  which 
the  bundles  of  muscle  fibers  radiate  outwards  to  its  edges. 
Any  shortening  of  these  fibers  evidently  diminishes  the 
diameter  of  the  diaphragm ;  and  because  of  its  form  (that 
of  a  dome  directed  upwards  into  the  thoracic  cavity), 
contraction  of  this  muscle  must  increase  the  size  of  the 
lower  thorax.1 

There  are  three  typical  modes  of  breathing :  (1)  The  pre^ 
dominantly  costal,  or  "  rib,"  breathing.  Here  the  diaphragm 
is  but  little  used.  It  is  the  type  characteristic  of  those  who 
impede  movements  of  the  lower  ribs  and  abdomen  with 
constricting  clothing,  such  as  tight  corsets.  (2)  The  pre- 
dominantly abdominal.  Here  the  ribs  are  little  used,  while 
the  diaphragm  does  most  of  the  work,  the  abdominal  mus- 
cles being  relaxed  so  that  the  belly  wall  has  its  maximum 
of  movement.  This  type  of  breathing  .involves  great  relax- 
ation of  tone  of  the  abdominal  muscles,  which  is  a  serious 

1  The  action  of  the  diaphragm  is  often  described  as  increasing  the  antero- 
posterior  (head  to  foot)  dimension  of  the  thorax  ;  but  this  can  happen  only 
when  the  diaphragm  is  free  to  descend,  and  it  can  descend  only  when,  by 
displacing  downwards  the  contents  of  the  abdominal  cavity,  it  causes  the 
well-known  respiratory  movements  of  the  abdominal  walls.  These  "abdomi- 
nal movements"  may,  however,  be  prevented  by  the  simultaneous  contrac- 
tion of  the  abdominal  muscles.  In  this  case  the  diaphragm  cannot  descend, 
and  its  contraction  can  only  raise  the  lower  ribs  to  which  it  is  attached. 
The  mechanism  in  these  two  methods  of  using  the  diaphragm  is  clear 
from  Fig.  80. 


RESPIRATION 


173 


disadvantage.  (3)  The  lateral  costal.  Here  the  abdominal 
muscles  act  at  the  same  time  as  the  ribs  and  the  diaphragm. 
This  form  of  breathing  produces  the  highest  pressures 
on  the  contents  of  the  abdominal  cavity  and  maintains  the 
tone  of  the  abdominal  walls  without  diminishing  the  effi- 
ciency of  the  oxygenation  of  the  blood.  It  also  forces  the 


FIG.  80.   Action  of  the  diaphragm  in  abdominal  and  in  lateral 
costal  breathing 

Solid  lines  represent  position  of  body  wall,  diaphragm,  and  ribs  during  expira- 
tion; dotted  lines,  the  same  during  inspiration.  The  left-hand  figure  represents 
abdominal  breathing,  the  diaphragm  becoming  more  convex,  displacing  down- 
ward the  abdominal  viscera  and  forcing  outward  the  abdominal  body  wall.  In 
the  lateral  costal  type  the  diaphragm  raises  the  lower  ribs,  and  the  abdominal 
walls  may  actually  move  inward,  owing  to  the  contraction  of  their  muscles 

use  of  the  upper  ribs  to  a  much  greater  extent  than  does 
the  predominantly  abdominal  type  of  breathing  (Fig.  80). 

It  is  seldom  that  one  or  another  of  these  types  is  used 
in  its  entirety,  and  the  advantages  of  one  form  over  another 
are  often  greatly  exaggerated.  The  following  statements 
may,  however,  be  taken  as  summing  up  the  essential 
practical  points. 


174  THE  HUMAN  MECHANISM 

1.  The    breathing  movements  should  be  such  as   to  use   all 
portions  of  the  lungs.    In  the  abdominal  type  there  is  little 
or  no  movement  of  the.  upper  thorax.     The  result  is  that 
the  apical,  or  upper,  Jobes  of  the  lungs  do  not  share  in  the 
enlargement  and  contraction  of  the  lungs;    they  are  poorly 
ventilated,    their    lymph    current  —  which    largely    depends 
upon  these  movements  —  becomes  sluggish,  and  because  of 
these  unfavorable   physiological  conditions  there  is  greater 
liability  to  disease.    More  than  60  per  cent  (some  observers 
claim,  as  many  as   80  per  cent)   of  the  beginnings  of  the 
lung  ravages  of  pulmonary  consumption  are  found  in  this 
portion  of  the  lung,  and  this  is  believed  to  be  due  to  the 
lack   of  movement  which  results   from  the   failure   to   use 
the  upper  thorax. 

2.  Actual  study  of  the  breathing  movements  in  people 
who  have  not  worn  constricting  clothing  indicates  that  the 
enlargement    of    the    thorax    in    inspiration    is    effected    by 
the   approximately  equal  action   of   the   diaphragm  and  of 
the  muscles  which  elevate  the  ribs. 

3.  The  abdominal  muscles  should  to  some  extent  contract 
with  the  diaphragm.    This  is  especially  important  in  those 
whose  occupation  is  more  or  less  sedentary,  as  it  is  the  most 
convenient  means  of  giving  to  these  muscles  the  use  which 
is  essential  to  the  maintenance   of  their   strength  and  the 
consequent  prevention  of  that  loss  of  tone  which  takes  away 
from  the  organs  of  the  abdominal  cavity  one  of  their  chief 
supports  (consult  Part  II,  Chap.  XVIII). 

4.  There  are  good  reasons  for  thinking  that  it  is  important 
to   develop  properly  the  muscles  of  the  upper  thorax  and 
especially  those  which  lie  in  the  triangle  between  the  root 
of  the  neck,  the  collar  bone,  and  the  shoulder  blade.    When 
these  muscles  are  not  developed,  especially  in  thin  people, 
the  wall  of  the  thorax  in  this  region  sinks  inward  during 
inspiration ;   under  these  circumstances  this  portion  of  the 
thorax  is  not  enlarged  during  inspiration,  the  apical  lobes  no 


EESPIRATION"  175 

longer  share  in  the  expansions  and  contractions  of  the  lungs, 
and  imperfect  ventilation  of  this  part  of  the  lung  results. 

10.  Secondary  effects   of   the   breathing  movements.    The 
student   will   now   be   better   able   to-  understand   the  part 
taken  by  the  breathing  movements  in  facilitating  the  return 
of  blood  and  lymph  to  the  heart.    The  enlargement  of  the 
thorax  during  inspiration  sucks  blood  and  lymph  in  toward 
the  great  veins  by  the  same  process  that  it  sucks  air  into 
the  lungs.    Especially  in  the  case  of  the  lymph  flow  is  this 
a  most  important  factor.    Moreover,  in  the  lymphatics  of  the 
lungs,  situated  as  they  are  entirely  within  the  thorax,  the 
movements  of  the  lungs  during  respiration  pump  the  lymph 
onwards  and  are  of  special  importance  in  this  respect.    Much 
of  the    invigorating   effect   of   muscular   exercise,   popularly 
ascribed  to  better  oxygenation  of  the  blood  and  tissues,  is 
really  attributable  to  the  greatly  improved  lymph  flow  from 
all   organs  which  results  from   the  deepened  respiration  in 
muscular  activity. 

11.  The  automatic  respiratory  center  and  its  regulation  by 
the  carbon  dioxide  of  the  blood.    The  muscles   of  the  dia- 
phragm  and   those   of  the  ribs,   like   the   biceps   and   other 
muscles  which  act  upon  the  skeleton,  are  stimulated  to  con- 
traction by  nervous  impulses  from  the  brain  and  spinal  cord. 
Every  movement  of  respiration  is  called  forth  and  regulated, 
in  accordance  with  the  needs  of  the  body  at  the  time,  by  the 
coordinated  action  of  a  number  of  nerve  cells.    Those  which 
are  most  intimately  concerned  with  respiration  are  found  in 
different  parts  of  the  central  nervous  system,  from  the  lower 
portion  of  the  brain  to  the  end  of  the  first  half  of  the  spinal 
cord,  inclusive ;   and  there  is  good  reason  for  thinking  that 
a  group  of  nerve  cells,  usually  known  as  the  respiratory  center, 
in  the  lower  portion  of  the  brain,  send  out  stimuli  to  those 
of  the  cord  and  through  them  excite  the  muscles  to  contract. 

The  respiratory  center,  like  the  heart  (see  p.  159),  is  auto- 
matic.   This  means  that  its  nerve  cells  periodically  (usually 


176  THE  HUMAN  MECHANISM 

eight  to  twenty  times  a  minute)  discharge  impulses  to  the 
respiratory  muscles  independently  of  any  stimulation  either 
by  afferent  nerves  or  by  other  means.  Like  the  beat  of  the 
heart,  however,  this  automatic  action  is  regulated  in  various 
ways.  A  dash  of  cold  water  on  the  skin  reflexly  changes 
the  character  of  respiration ;  coughing  and  sneezing  are  simi- 
larly examples  of  reflex  modification  of  the  breathing  move- 
ments ;  during  vigorous  muscular  activity  the  change  in 
composition  of  the  blood  by  the  addition  of  waste  products 
deepens  and  quickens  the  breathing ;  last,  but  not  least,  one 
of  the  most  important  discoveries  of  recent  years  has  shown 
that  the  carbon  dioxide  of  the  arterial  blood  going  to  the 
respiratory  center  is  a  most  important  agent  in  regulating 
the  automatic  activity  of  the  center.  No  sooner  does  the 
carbon  dioxide  of  the  blood  increase  than  the  center  dis- 
charges more  powerfully,  thus  deepening  the  breathing.  An 
increase  of  from  3  to  4  per  cent  in  the  carbon  dioxide 
of  the  arterial  blood  doubles  the  quantity  of  air  breathed 
per  minute.  From  this  it  is  evident  that  the  high  content 
of  this  gas  in  arterial  blood  (see  p.  166)  serves  the  very 
important  function  of  adjusting  the  work  of  the  center  to 
the  needs  of  the  body.  Whenever,  for  any  cause,  the  respira- 
tory movements  no  longer  adequately  ventilate  the  lungs  — 
so  that  carbon  dioxide  discharged  upon  the  blood  in  its 
course  through  the  body  is  not  completely  removed  in  the 
lungs  —  the  consequent  increase  of  this  gas  in  the  arterial 
blood  excites  the  center  to  greater  activity,  with  a  resulting 
increase  of  breathing  and  more  efficient  ventilation  of  the 
lungs.  We  may  recall,  in  this  connection,  the  warning  given 
in  Chapter  VI  against  supposing  that  a  "  waste  product " 
of  the  activity  of  one  organ  is  necessarily  harmful,  for 
carbon  dioxide  is  the  chief  waste  of  the  body;  yet  it  is 
most  important  that  the  amount  usually  present  in  arterial 
blood  be  maintained.  Only  the  excess  above  this  amount 
is  injurious. 


RESPIRATION  177 

12.  The  circulation  as  an  essential  part  of  the  mechanism 
of  respiration.  The  consumption  of  oxygen  and  the  produc- 
tion of  carbon  dioxide  thus  involve  an  interchange  of  these 
gases  between  the  blood  and  the  tissues  (internal  respiration) 
on  the  one  hand,  and  between  the  blood  and  the  air  in  the 
lungs  (external  respiration)  on  the  other.  But  to  carry  out 
these  gaseous  exchanges  a  third  factor  is  obviously  necessary, 
namely,  a  means  of  communication  between  the  two,  so  that 
the  oxygen  absorbed  in  the  lungs  may  be  carried  to  the 
tissues,  and  the  carbon  dioxide  produced  in  the  tissues  be 
carried  back  to  the  lungs.  This  communication  is  provided, 
as  has  been  shown  in  earlier  chapters,  by  the  circulation, 
which  thus  becomes  an  essential  part  of  the  respiratory 
mechanism. 

We  have  already  seen  that  under  the  most  varying  con- 
ditions 100  cc.  of  arterial  blood  always  contain  approxi- 
mately 20  cc.  of  oxygen  and  38  cc.  of  carbon  dioxide  and 
that  this  is  practically  all  the  oxygen  this  amount  of  blood 
can  hold.  From  this  it  follows  that  so  long  as  the  amount 
of  blood  pumped  by  the  heart  in  a  given  time  remains  con- 
stant, no  more  oxygen  will  be  carried  to  the  tissues,  even 
if  we  breathe  more  deeply.  In  other  words,  increased  ventila- 
tion of  the  lungs  without  any  accompanying  increase  in  the  rate 
and  force  of  the  heart  beat  will  not  supply  more  -oxygen  to  the 
tissues.  The  beat  of  the  heart  is  as  important  to  proper  tis- 
sue respiration  as  are  the  breathing  movements ;  and  we  find 
accordingly  that  these  two  events  are  closely  coordinated. 
Greatly  increased  tissue  respiration  invariably  carries  along 
with  it  increased  work  on  the  part  of  the  heart. 

A  large  number  of  measurements  of  the  respiratory  ex- 
changes l  under  different  conditions  and  activities  of  our 
life  has  shown  that  these  are  increased  by  the  taking  of 
food,  by  exposure  to  cold,  by  awaking  from  sleep,  and, 
above  all,  by  muscular  activity.  Exposure  to  cold  acts  by 

1  That  is,  oxygen  absorbed  and  carbon  dioxide  discharged  in  a  given  time. 


178  THE  HUMAN  MECHANISM 

causing  us  to  move  about  more  briskly,  or,  if  we  do  not, 
by  causing  us  to  shiver,  so  that  this  really  becomes  a  case 
of  muscular  activity.  The  same  thing  is  true  of  awakening 
from  sleep.  We  may  therefore  make  the  general  statement 
that  muscular  activity  is  the  one  important  agent  of  life 
which  increases  tissue  respiration. 

And  this  increase  is  at  times  very  great.  Even  the  mus- 
cular activity  necessary  to  maintain  the  erect  position  in 
sitting  and  standing,  as  compared  with  the  complete  relax- 
ation of  sleep,  doubles  the  gaseous  exchange ;  gentle  exercise 
(a  walk  of  three  miles  an  hour)  more  than  doubles  that  of 
rest ;  and  vigorous,  yet  by  no  means  excessive,  exercise  will 
increase  it  tenfold.  These  increases  mean  corresponding, 
though  not  absolutely  proportionate,  demands  on  the  heart 
and  emphasize  the  importance  of  keeping  that  organ  in  an 
efficient  working  condition.  Breathlessness,  for  example, 
usually  indicates,  in  part  at  least,  that  the  heart  fails  to 
respond  properly  to  the  demands  made  upon  it,  these  de- 
mands being  greater  than  it  can  meet  without  undue  fatigue; 
it  is  a  warning  that  we  are  pushing  the  heart  too  hard,  a 
warning  which  we  will  do  well  to  heed.  Generally  it  is 
also  a  warning  that  we  are  not  getting  sufficient  muscular 
activity;  the  heart  fails  to  meet  the  emergency  of  some 
unusual  exertion  because  all  along  it  has  not  been  kept  in 
proper  training;  so  that  while  we  should,  as  stated,  heed 
the  warning  not  to  push  the  heart  so  hard  for  the  time 
being,  we  should  also  act  upon  the  equally  important  warn- 
ing that  it  needs  practice  or  training  —  a  training  which  can 
be  given  only  by  reasonable,  regular,  muscular  activity. 

The  training  of  muscular  activity  is  therefore  not  only  a 
training  of  the  muscles  but  also  of  the  heart.  But  this  is 
not  all.  The  work  of  the  circulatory  and  respiratory  mecha- 
nisms must  be  adjusted  or  coordinated,  the  one  to  the  other. 
When,  for  example,  the  deepened  breathing  movements 
accompanying  muscular  activity  rush  the  blood  back  more 


EESPIRATION  179 

rapidly  to  the  heart  (p.  148),  it  becomes  necessary  for  the 
heart  to  adjust  the  character  of  its  beat  to  the  new  condi- 
tions ;  and  this  adjustment  is  the  work  of  the  nervous  sys- 
tem. Time  is,  however,  required  to  make  the  adjustment,  so 
that  it  is  wise  to  "  warm  up "  gradually  to  more  vigorous 
work.  We  can  also  understand  how  by  physical  training 
this  process  of  adjustment  comes  to  be  shortened,  for  we 
have  not  only  trained  the  heart  by  giving  it  more  work  to 
do  but  we  have  also  trained  those  portions  of  the  nervous 
system  which  regulate  its  beat. 


CHAPTER  XI 
EXCBETION 

1.  The  organs  of  excretion.    The  student  now  realizes  that 
the  work  of  the  body  is  accompanied  by  the  production  of 
wastes  and  also  understands  the  necessity  for  their  removal. 
The  most  abundant  waste  product  of  the  body,  carbon  dioxide, 
is  a  gas  and  is  excreted  by  the  lungs ;  others,  notably  urea 
and  other  waste  products  of  the  proteins,  are  dissolved  solids 
and  are    removed  from  the  blood  to  some    extent  by  the 
intestine  and   the  sweat  glands  of  the  skin,  but  chiefly  by 
the  kidneys. 

A  number  of  organs  thus  perform  the  work  of  excretion, 
but  four  of  them  —  namely,  the  lungs,  the  kidneys,  the  in- 
testine, and  the  skin  —  are  of  greater  importance  than  all 
others.  Of  these  four  the  lungs  and  kidneys  are  far  more 
important  than  the  intestine,  and  all  three  of  these  are  more 
important  than  the  skin. 

2.  Essential  and  incidental  excretion  by  organs.    An  organ 
may  be  essential  to  the  proper  removal  of  a  given  waste,  or 
it  may  remove  the  waste  product  only  incidentally  in  per- 
forming  its   essential  functions.     Thus  the  skin   removes  a 
small  amount  of  carbon  dioxide  from  the  body  merely  be- 
cause a  certain  amount  of  this  gas  diffuses  from  the  blood 
as   it   flows   through   the   skin.    It   is   not  necessary   to   the 
health  of  the  body  that  the  skin  should  excrete  this  carbon 
dioxide,  for  the  lungs  are  quite  capable  of  doing  the  work 
and  would  do  so  if  for  any  reason  such  excretion  through 
the  skin  were  prevented.    Without  the  lungs,  on  the  other 
hand,  the  carbon  dioxide  would  rapidly  accumulate  in  the 

180 


EXCKETION  181 

blood  and  cause  death.  The  lungs  are  essential  to  the  re- 
moval of  this  waste;  the  skin  is  not.  Similarly,  the  perspi- 
ration contains  small  amounts  of  urea  and  other  wastes 
which  are  removed  in  large  quantities  by  the  kidneys.  It  is 
not  necessary  that  the  skin  should  remove  any  of  these,  for 
the  healthy  kidney  can  and  does,  when  necessary,  remove 
them.  Small  quantities  of  them  appear  in  the  perspiration 
because  they  are  in  the  blood  from  which  the  perspiration 
is  formed  and  because  the  cells  of  the  sweat  glands  allow 
them  to  pass  through,  just  as  the  skin  allows  the  passage 
of  carbon  dioxide.  . 

These  considerations  are  of  practical  importance  in  the 
hygiene  of  the  skin.  It  is  not  necessary  to  induce  perspi- 
ration merely  to  remove  waste  products  from  the  body.  If 
the  human  skin,  like  that  of  the  cat  and  the  dog,  contained 
no  sweat  glands,  the  waste  products  would  be  thoroughly 
removed;  and  in  cold  weather,  when  no  perspiration  is 
secreted,  the  excretion  of  waste  is  as  complete  as  when  in 
warm  weather  perspiration  is  abundantly  secreted.  On  the 
other  hand,  perspiration,  though  not  secreted  to  rid  the  body 
of  wastes,  nevertheless  contains  wastes  which  accumulate 
upon  the  skin.  Hence  the  need  of  bathing,  both  as  a  matter 
of  health  and  of  decency. 

The  chief  wastes  leaving  the  body  and  their  main  chan- 
nels of  excretion  are  given  in  the  following  table,  incidental 
excretions  being  given  in  italics: 

Lungs  :  carbon  dioxide,  water. 

Kidneys :  urea,  uric  acid,  and  other  compounds,  salts,  water. 
Intestine  :  bile  pigments,  nitrogenous  compounds,  etc. 
Skin :  urea,  etc.,  salts,  water. 

The  structure  and  action  of  the  lungs  and  intestine  have 
already  been  described,  so  that  we  have  left  for  study  the 
kidneys  and  the  skin. 

3.  Structure  of  the  kidneys.  Each  kidney  is  a  bean-shaped 
gland  whose  duct,  the  ureter,  runs  to  the  urinary  bladder.  As 


182 


THE  HUMAN  MECHANISM 


-  Vena  Cava 

-  Ureter 


the  ureter  enters  the  kidney  at  the  center  of  the  depression 
in  that  organ  it  expands  to  form  a  basin,  known  as  the  pelvis 
of  the  ureter.  Into  this  basin  open  the  hundreds  of  glandular 
tubules  of  which  the  bulk  of  the  kidney  is  composed.  Each 
tubule,  like  the  alveolus  and  ducts  of  the  gland  described  in 
Chapter  III,  consists  of  a  single  layer  of  cells,  which  separate 
the  blood  and  lymph  from  the  lumen  of  the  tubule ;  and  the 

formation  of  urine  by  the  kidney 
is  essentially  an  act  of  secretion. 
4.  The  secretion  of  urine. 
The  urine  is  secreted  continu- 
ously from  the  blood,  at  one 
time  more  rapidly  than  at  an- 
other, but  under  normal  condi- 
tions never  ceasing  altogether. 
Passing  down  the  tubules,  it 
collects  in  the  upper  portion  of 
the  ureter,  and  successive  peri- 
staltic waves  carry  it  from  this 
point  to  the  urinary  bladder,  an 
organ  with  muscular  walls  in 
which  the  urine  accumulates 
and  from  which  it  is  from  time 
to  time  discharged. 

In  one  very  important  re- 
spect, however,  secretion  by  the 
kidney  presents  a  sharp  contrast  to  secretion  by  the  stomach 
and  the  submaxillary  gland.  While  an  adequate  blood  supply 
to  the  two  latter  glands  accompanies  secretion  and,  indeed, 
is  necessary  to  maintain  the  secretion  for  any  length  of  time, 
yet  these  glands  secrete  only  as  they  are  stimulated  to  ac- 
tivity by  their  nerves ;  merely  increasing  their  blood  supply 
does  not  produce  increased  secretion.  In  the  case  of  the 
kidney  there  seem  to  be  no  secretory  nerves,  and  the  activity 
of  the  gland  seems  to  be  determined  to  a  large  extent  ly  the 


FIG.  81.    Dorsal  aspect  of  the  kid- 
neys, ureter,  urinary  bladder,  and 
abdominal  aorta  and  vena  cava 


EXCRETION 


183 


quantity  of  blood  flowing  through  it.  Anything  which  increases 
this  quantity  of  blood  increases  the  quantity  of  urine  secreted; 
anything  which  diminishes  it  lessens  the  amount  of  urine 
secreted. 

In  the  everyday  experience  of  healthy  people  the  activity 
of  the  kidneys  is  chiefly  affected  by  three  things ;  namely, 
(1)  external  temperature  —  more 
urine  is  secreted  on  a  cold  than  on 
a  warm  day;  (2)  the  quantity  of 
water  drunk ;  and  (3)  the  quantity 
of  food,  and  especially  of  protein 
food,  eaten.  All  three  of  these 
agents,  however,  produce  their  re- 
sults, largely  if  not  entirely,  because 
of  their  influence  upon  the  blood 
flow  through  the  kidney.  Thus  ex- 
posure of  the  skin  to  cold  causes  a 
constriction  of  the  arterioles  of  the 
skin  and  a  compensating  dilation  of 
those  of  internal  organs,  the  kid- 
neys included.  More  blood  flows 
through  the  kidneys  and  more  urine 

is    Secreted.      Much    the    Same    thing      open,  on  the  papillae  (B,  B},  into 

is  true  of  the  absorption  of  water      "  pel™(c'> o£  the  uretl 
and  of  protein  food,  for  both  these  conditions  cause  a  widen- 
ing of  the  arterioles  of  the  kidney. 

Changes  in  the  quantity  of  the  urine  secreted  are,  generally 
speaking,  only  changes  in  the  amount  of  water  rather  than  in 
the  amount  of  urea  and  other  dissolved  wastes.  Certain  con- 
stituents of  the  urine,  however,  are  not  very  soluble,  so  that  it 
is  not  well  to  have  water,  the  only  solvent  of  these  substances 
in  the  urine,  unduly  diminished.  A  scanty  secretion  of  urine 
during  the  day  is,  in  general,  a  distinct  indication,  especially 
in  warm  weather,  that  insufficient  water  is  being  taken.  Many 
persons  drink  too  little  water  rather  than  too  much. 


FIG.  82.    Vertical  section  of 
the  kidney.     Diagrammatic 

The  tubules  (A)  of  the  gland 


184 


THE  HUMAN  MECHANISM 


5.  The  structure  of  the  skin.  The  skin  is  an  organ  which 
performs  several  functions,  the  most  important  being  (1)  that 
of  protecting  the  underlying  structures 
from  drying  and  mechanical  injury  ; 
(2)  that  of  assisting  in  maintaining  the 
constant  internal  temperature  of  the 
body  ;  and  (3)  that  of  receiving  the  ex- 
ternal stimuli  of  pressure,  heat,  and  cold. 
Incidentally,  as  we  have  seen,  the  skin 
is  an  organ  of  excretion.  We  may  there- 
fore describe  its  structure  and  excretory 
function  in  this  connection,  reserving 
the  study  of  its  other  functions  for 
Chapters  XII  and  XIV. 

The  skin  consists  of  an  outer  layer, 
the  epidermis,  and  an  inner  layer,  the 
dermis,  cutis,  or  corium.  The  clermis 
consists  of  connective  tissue  richly  sup- 
plied with  blood  vessels,  lymphatics, 
and  nerve  fibers,  together  with  sense 
organs  of  touch.  The  fiber  bundles  of 
the  connective  tissue  are  most  dense 
near  the  epidermis;  in  the  deeper  por- 
tions the  network  is  loose  and  the  lymph 
FIG.  83.  Cross  section  spaces  larger,  the  connective  tissue  of 
the  dermis  passing  insensibly  into  that  of 
the  subcutaneous  connective  tissue. 

The  cells  of  the  more  open  portions 
sweat  gland;  D,  dermis;     of  the  dermal  network,   and  especially 

E,  subcutaneous  connec-       ,  , 

The    those  of  the  subcutaneous  tissue,  store 

within    their    CVtO- 
.       J 

plasm.  The  subcutaneous  tissue,  indeed, 
is  one  of  the  most  important  organs  in  the  body  for  the 
storage  of  fat.  Connective  tissue  in  which  large  amounts  of 
fat  are  stored  is  known  as  adipose  tissue  (see  p.  223). 


E 


of  skin 

A,  horny  layer  of  epi- 
dermis; B,  deeper  layer 
of  epidermis ;  C,  duct  of 


tive  tissue   (p.  7). 

blood  vessels  are  injected      up    more    or    }egs 
to  show  black.  Cf.Fig.89 


EXCRETION 


185 


The  outer  surface  of  the  dermis  is  not  flat,  but  contains 
rooundlike  projections  known  as  papillce,  which  project  into 
the  overlying  epidermis.  Some  of  these  papillae  contain  nerve 
endings  of  the  sense  of  touch,  while  others  contain  capillaries, 
which  are  found  also  in  other  portions  of  the  dermis.  The 
dermis  is  the  vascular  organ  of  the  skin,  blood  vessels  being 
entirely  absent  from  the  epider- 
mis (see  Figs.  86,  89). 

The  epidermis  consists  of 
many  layers  of  cells,  the  num- 
ber of  layers  being  very  great  — 
a  hundred  or  more  on  the  palms 
of  the  hands  and  the  soles  of 
the  feet;  in  other  places  less 
exposed  to  pressure  or  friction 
they  may  not  exceed  twenty. 
The  deeper  cells  (that  is,  those 
nearer  the  dermis)  are  alive  and 
in  process  of  active  growth  and 
multiplication.  The  outer  layers, 
which  are  further  from  the  der- 
mis with  its  blood  supply  and 
nearer  the  surface  with  its  ex- 
posure to  drying,  degenerate  and 
are  gradually  transformed  into 
dead,  flattened  horny  scales 
which,  packed  together,  form  the 
horny  layer.  These  scales  are 
being  constantly  rubbed  off  and  their  loss  made  good  by  the 
growth  and  multiplication  of  the  living  cells  beneath.  Such  a 
covering  or  lining  is  well  fitted  for  surfaces  which  are  exposed 
to  friction  or  drying,  and  we  accordingly  find  that  the  mouth, 
the  part  of  the  pharynx  used  in  swallowing,  the  oesophagus, 
and  the  rectum  are  lined  with  the  same  tissue.  The  endings 
of  nerve  fibers  are  found  in  the  lower  layers  of  the  epidermis. 


FIG.  84.    Hair  and  hair  follicle 

A,  horny  layer  of  epidermis.  B, 
layer  of  living,  growing  cells  ex- 
tending (B')  into  the  hair  follicle,  at 
the  bottom  of  which  it  forms  the 
mass  of  growing  cells  E  over  the 
papilla  (P)  with  its  knot  of  capil- 
laries; the  growth,  multiplication, 
and  transformation  of  these  cells 
into  horny  fibers  forms  the  shaft  of 
the  hair,  D.  C,  capillaries  in  the 
dermis.  S,  a  sebaceous  gland  dis- 
charging its  oily  secretion  (0)  into 
the  follicle  to  lubricate  the  hair  and 
the  horny  layer  of  the  skin 


186 


THE  HUMAN  MECHANISM 


The  hairs,  the  sweat  glands,  and  the  nails  are  modified 
portions  of  the  epidermis.  Of  these  the  hairs  and  the  sweat 
glands  are  of  sufficient  importance  to  m^rit  some  description. 

6.  Structure  of  a  hair  and  a  hair  follicle.    A  hair  grows  from 
the  bottom  of  a  pit,  the  hair  follicle,  which  extends  downward 

into  the  dermis  or  even  into  the 
subcutaneous  tissue.  Microscopic 
examination  shows  that  this  fol- 
licle is  lined  with  a  continuation 
of  the  epidermis,  just  as  a  gland 
of  the  stomach  or  intestine  is 
lined  by  an  ingrowth  of  the  cells 
of  its  surface.  At  the  bottom  of 
the  follicle  is  a  papilla,  and  the 
hair  which  grows  out  from  this 
papilla  to  the  surface  bears  to 
the  cells  of  the  papilla  the  same 
relation  that  the  horny  layer  of 
the  epidermis  bears  to  the  similar 
underlying  cells.  We  accord- 
ingly find  that  the  hair  is  com- 
posed of  horny  scales  closely 
pressed  together  into  the  well- 
known  threadlike  structure. 

Opening  into  the  hair  follicle, 
one  or  more  sebaceous  glands  dis- 
charge an  oily  secretion  which 
lubricates  the  hair  and  the  horny  layer  of  the  epidermis,  and 
so  prevents  drying  and  chapping  (Figs.  84  and  85). 

7.  The  sweat  glands  are  tubular  prolongations  of  the  epi- 
dermis through  the  dermis  into  the  subcutaneous  tissue.  Here 
the  tube  becomes  much  coiled,  forming  the  secreting  recess, 
which  is  richly  supplied  with  blood  vessels  and  also  receives 
nerves.    It  is  a  simple  tubular  gland  formed  as  an  ingrowth 
from  the  epidermis  (see  Figs.  86  and  89). 


FIG.  85.  Magnified  section  of  the 

lower  portion  of  a  hair  and  hair 

follicle 

A,  membrane  of  the  hair  follicle, 
cells  with  nuclei  and  pigmentary 
granules ;  B,  external  lining  of  the 
root  sheath;  C,  internal  lining  of 
the  root  sheath;  D,  cortical  or 
fibrous  portion  of  the  hair  shaft; 
JS,  medullary  portion  (pith)  of 
shaft;  F,  hair  bulb,  showing  its 
development  from  cells  from  A 


EXCRETION 


187 


8.  The  secretion  of  the  perspiration,  like  the  secretion  of 
the  gastric  juice,  is  under  the  control  of  the  nervous  system. 
When  the   nerves  going  to   the   sweat  glands   of    a   given 
area  of  skin  are  cut  or  otherwise  injured,  the  secretion  of 
perspiration  ceases   over  that  area;   and  the   appearance  of 
cold   beads    of   perspiration    as   the  result  of   fright   shows 
how  events  taking  place  in  the  nervous 

system  may  excite  these  glands  to  activity 
apart  from  the  presence  of  their  usual 
stimuli  —  the  application  of  heat  to  the 
skin  and  the  liberation  of  heat  within  the 
body  by  muscular  and  other  activities. 
The  distinction  should  be  made  between 
the  so-called  "  sensible  "  and  "  insensible  " 
perspiration,  the  latter  name  being  given 
to  the  perspiration  the  water  of  which 
evaporates  as  fast  as  secreted  ;  the  former 
to  that  which  does  not  evaporate  so  rapidly 
and  hence  remains  for  a  time  on  the  sur- 
face of  the  skin.  When  the  water  evapo- 
rates, the  dissolved  solids  (salts,  urea,  and 
other  compounds)  remain  behind  on  the 
skin. 

9.  Value  of  profuse  perspiration  in  the 
care  of  the  skin.    While  the  skin  is  not 

.-,  p  ,.          ,-,  Note  the  coiled  form 

primarily  an  organ  of  excretion,  the  per-  of  the  tube  in  the 
spiration  contains  a  certain  amount  of 
waste  substances  and  salts,  which  are  left 
by  the  evaporation  of  the  water  upon  the  surface  and,  to 
some  extent,  in  the  mouths  of  the  ducts  of  the  sweat  glands  ; 
this  is  especially  the  case  when  evaporation  takes  place  about 
as  rapidly  as  the  perspiration  is  discharged.  When  the  secre- 
tion of  perspiration  is  more  abundant,  as  during  muscular 
work,  or  at  very  high  temperatures,  or,  in  general,  where  it 
does  not  evaporate  as  rapidly  as  discharged,  the  accumulation 


FIG.  86.  Sweat  gland 
(slightly  magnified) 


subcutaneous    tissue. 


188  THE  HUMAN  MECHANISM 

of  solids  in  the  ducts  of  the  glands  is  washed  out.  For  this 
reason  a  vigorous  perspiration  followed  by  a  bath  is  a  useful 
hygienic  measure  in  the  care  of  the  skin,  although  it  is  not 
necessary,  as  is  sometimes  supposed,  in  order  to  secure  the 
efficient  elimination  of  wastes  from  the  blood. 

10.  The  skin  as  an  organ  of  absorption.  While  it  is  true 
that  water  as  perspiration  may  readily  find  its  way  out 
through  the  skin,  such  escape  is  effected  chiefly  by  the 
sweat  glands,  which  are  under  the  strict  control  of  the  nerv- 
ous system.  Apart  from  this  the  skin  is  virtually  water- 
tight; and,  oiled  as  it  is  by  the  secretion  of  the  sebaceous 
glands,  it  serves  both  to  keep  in  the  water,  which  forms  so 
important  a  part  of  the  tissues,  and  also  to  keep  out  water 
which  might  otherwise  soak  into  the  body,  as,  for  example, 
during  bathing.  This  waterproof  characteristic  also  makes  it 
next  to  impossible  for  us  to  absorb  food  materials  by  way  of 
the  skin.  A  "  milk  bath"  may  be  at  times  useful  in  the  care 
of  the  skin,  because  the  fat  or  oil  of  the  milk  may  supply 
any  deficiency  in  the  sebaceous  secretion  and  so  insure 
lubrication  of  the  epidermis ;  but  it  cannot  be  regarded  as 
a  means  of  supplying  food  to  the  body. 


CHAPTER  XII 

THERMAL  PHENOMENA  OF  THE  BODY 
A.  THE  'CONSTANT  TEMPERATURE 

1.  The   normal   temperature.     No    characteristic    of    the 
human    mechanism   is   more    remarkable    than    its    constant 
temperature.    Whether  we  are  awake  or  asleep,  by  night  or 
by  day,  at  work  or  at  rest,  at  home  or  abroad,  in  summer 
or  in  winter,  in  the  tropics  or  in  the  polar  regions,  in  subter- 
ranean caves  or  on  lofty  mountain  peaks,  the  temperature  of 
healthy  human  beings  is  always  nearly  the  same.    So  steady 
is  this  temperature  that  an  increase  or  decrease  of  two  or 
three  degrees  gives  just  cause  for  anxiety,  and  a  change  of 
seven  or  eight  degrees  is  looked  upon  with  alarm. 

In  many  modern  laboratories  constant  temperatures  are 
obtained  by  the  use  of  a  thermostat,  the  apparatus  of  which 
is  visible  and  easily  understood ;  but  no  such  special  appa- 
ratus regulates  the  constant  temperature  of  the  human  body, 
and  we  have  rather  to  seek  an  explanation  in  the  coordi- 
nated activities  of  organs  already  familiar,  such  as  muscles, 
skin,  blood  vessels,  and  especially  the  all-controlling  nervous 
system. 

2.  Temperature    and    chemical    changes.     Every   chemical 
reaction    takes    place    more    readily    under    some    external 
physical   conditions    than    under   others,    and    among    these 
conditions  none  is  more  important  than  temperature.    This 
fact  is  illustrated  in  the  case  of  the  enzymes.    At  the  freez- 
ing point  saliva  exerts  no  action  upon  starch  paste ;   as  the 
temperature  rises,  the  activity  of  the  enzyme  increases  up 
to  a  certain  point  and  then  diminishes  more  or  less  rapidly 

189 


190  THE  HUMAN  MECHANISM 

until  a  point  is  finally  reached  at  which  its  peculiar  chemi- 
cal properties  are  destroyed. 

3.  Temperature  and  vital  activities.    When  we  come  to 
the  activities  of   living  cells  —  activities  which,   it  will  be 
recalled,  depend  on  chemical  changes  —  precisely  the  same 
thing  holds  true  and  in  so  striking  a  manner  as  to  create  a 
widespread  but  erroneous  impression  that  this   dependence 
upon  temperature  is  peculiarly  characteristic  of  living  things. 
The  one-celled  animal,  amoeba,  moves  about  more  actively 
and  digests  more  food  at   20°  C.   than   at   10°  C. ;  bacteria 
grow  more  rapidly  at  the  room  temperature  than  near  the 
freezing  point;  the  pitch  of  the  note  made  by  a  cricket  rises 
with  the  temperature,  indicating  that  the  movements  of  the 
wing  covers  which  produce  the  sound  are  being  made  more 
rapidly ;  and  in  the  winter  sleep  of  hibernating  animals  we 
have  a  beautiful  example  of  the  decline  of  vital  activities 
with  the  fall  of  external  temperature. 

Nor  are  the  living  cells  of  the  human  body  exceptions  to 
this  rule.  The  rate  of  the  heart  beat  varies  directly  with  the 
temperature  of  the  blood,  and  the  character  of  the  breathing 
movements  is  influenced  by  the  same  cause;  a  cooled  muscle 
contracts  more  slowly,  a  cooled  gland  secretes  less  abun- 
dantly. If  the  temperature  of  the  body  itself  falls,  every 
vital  activity  is  depressed,  and  death  itself  may  result  from 
undue  cooling. 

4.  The  constant  temperature  of  the  body.    This  depression 
of   nervous,   muscular,  and  glandular  activity  results,  how- 
ever, only  from  a  fall  of  the  temperature  of  the  body,  not  of 
that  of  the  surrounding  air  or  other  medium.     These  two 
things  are  by  no  means  the  same,  as  may  be  readily  seen 
from  the  fact  that  a  thermometer  placed  in  the  mouth  indi- 
cates almost  the  same  temperature  of  the  body  on  warm  and 
on  cold  days ;  even  while  we  are  shivering  with  cold  the 
thermometer  gives  about  the  same  reading  as  when  we  are 
enjoying  the  warmest  summer  weather.    The  temperature  of 


THERMAL  PHENOMENA  OF  THE  BODY         191 

the  body  remains  nearly  constant,  regardless  of  changes  in 
the  temperature  of  the  air  around  it. 

We  have  only  to  appeal  to  experience  to  see  that  this  is 
not  the  way  in  which  lifeless  matter  generally  behaves;  a 
stone,  the  earth,  a  piece  of  iron  is  warmer  on  a  warm  day 
and  colder  on  a  cold  day;  in  general,  lifeless  things  take  the 
temperature  of  the  medium  in  which  they  are  placed,  and  this 
is  one  of  the  fundamental  principles  of  physics.  Nor  do  most 
living  things  act  differently;  the  temperature  of  a  plant  or 
a  tree,  of  an  earthworm,  a  frog,  a  turtle,  a  snake,  does  not 
differ  greatly  from  that  of  its  surroundings.  It  is  only  birds 
and  mammals  which  show  this  remarkable  power  of  maintain- 
ing an  approximately  constant  body  temperature  notwith- 
standing wide  limits  of  change  in  that  of  the  surrounding  air. 
Such  animals  are  known  as  warm-blooded  because  they  are 
usually  warmer  than  surrounding  objects;  those  animals 
which  do  not  thus  maintain  a  constant  temperature,  on  the 
other  hand,  are  known  as  cold-blooded.1 

It  is  clear  that  the  power  to  maintain  a  constant  body 
temperature  is  of  the  utmost  importance  in  enabling  an  ani- 
mal to  counteract  the  varying  conditions  of  climate.  Were 
it  not  for  this  power,  man  would  be  a  hibernating  animal; 
with  the  coming  of  winter  all  his  activities  would  gradually 
be  slowed  down  and,  long  before  our  rivers  and  ponds  had 
begun  to  freeze,  all  business,  industrial  life,  and  intellectual 
life  would  come  to  a  standstill;  it  would  not  be  possible 
for  the  human  race  to  people  every  zone  of  the  earth  —  the 
shores  of  Alaska  or  Iceland  as  well  as  the  banks  of  the 
Ganges  or  the  Amazon. 

5.  The  temperature  of  the  body  not  absolutely  constant. 
The  term  "  constant "  as  applied  to  the  temperature  of 

1  A  cold-blooded  animal  exposed  to  a  temperature  of  99°  F.  is  as  warm 
as  a  warm-blooded  animal.  Such  animals  are  so  called  because  they  usually 
feel  colder  when  handled  than  do  warm-blooded  animals  ;  but  this  is  merely 
because  the  temperature  of  the  air  (which  is  also  their  temperature)  is  usually 
lower  than  the  temperature  of  warm-blooded  animals. 


192  THE  HUMAN  MECHANISM 

warm-blooded  animals  is  not,  however,  to  be  taken  too  literally. 
No  animal  has  an  absolutely  constant  temperature.  In  the  first 
place,  there  are  slight  variations  from  time  to  time  under  the 
changing  conditions  of  life.  The  temperature  is  higher  by 
from  one  to  four  degrees  during  muscular  activity  than 
during  rest;  it  varies  during  the  day,  being  highest  in  the 
afternoon  and  lowest  in  the  small  hours  of  the  morning;  it 
is  often  raised  half  a  degree  or  more  by  taking  food,  and 
marked  changes  of  surrounding  temperature  may  cause  a 
change  of  one  degree  or  even  more  in  that  of  the  body. 
These  changes  between  97.5°  and  99.5°  F.  are  of  everyday 
occurrence  and  are  entirely  normal;  so  that  when  we  speak 
of  the  temperature  of  the  body  being  constant  we  mean  that 
it  varies  only  within  narrow  limits  or  that  it  is  constant  in 
comparison  with  that  observed  in  cold-blooded  animals. 

6.  The  temperature  of  different  organs.  Nor  is  this  all; 
some  parts  of  the  body  have  a  higher  temperature  than 
others.  Thus  the  temperature  of  the  liver  is  often  as  high 
as  107°  F. ;  that  of  the  muscles  varies  between  99°  and 
105°  F. ;  that  of  the  blood  in  the  right  side  of  the  heart  is 
usually  a  degree  or  so  higher  than  that  of  the  blood  in  the 
left  side.  But  it  is  in  the  skin  that  we  meet  with  the  widest 
variations  from  the  general  average.  Everyone  knows  that 
on  a  very  cold  day  the  temperature  of  the  skin  may  be  far 
below  98.6°  F. ;  indeed,  the  experience  of  "  frosted  "  ears  or 
feet  shows  that  at  times  cutaneous  temperature  may  descend 
to,  or  even  below,  the  freezing  point  itself;  and  it  is  very 
exceptional  indeed  when  the  skin  temperature  is  above  92° 
or  93°  F.,  even  on  very  hot  summer  days.  These  variations 
are  due  to  the  fact  that  the  skin  is  the  organ  which  is 
immediately  exposed  to  the  changing  environment  and 
hence  peculiarly  subject  to  cooling  influences.  It  is  there- 
fore customary  to  distinguish  between  an  outer  body  zone 
of  variable  temperature  and  the  more  constant  temperature 
of  internal  organs. 


THERMAL  PHENOMENA  OF  THE  BODY         193 

7.  Measurement  of  the  body  temperature.    The  great  equal- 
izer of  the  body  temperature  is  the  blood.    Blood  which  has 
flowed   through  the   skin  comes  away  cooled ;   that  which 
comes  from  an  organ  like  the  liver  or  a  working  muscle,  in 
which  active  oxidations  or  other  chemical  changes  have  taken 
place,  is  heated.    In  the  great  veins  and  in  the  heart  the 
warmer  blood   is  mixed   with   the  cooler,    and   an    average 
temperature  of  the  arterial  blood  results.    It  is  this  average 
temperature  of  the  arterial  blood  flowing  to  the  organs  that 
is  approximately  constant. 

When  this  blood  flows  for  a  time  through  an  organ  which 
is  itself  not  producing  heat  and  is  at  the  same  time  protected 
from  loss  of  heat,  the  organ  ultimately  takes  on  the  tempera- 
ture of  the  blood;  so  that  by  measuring  the  temperature  of 
such  an  organ  we  get  the  temperature  of  the  blood  itself. 
It  is  customary  to  take  the  temperature  in  the  mouth,  the 
bulb  of  the  thermometer  being  placed  under  the  tongue  and 
the  lips  kept  closed.  Subject  to  the  variations  mentioned 
above,  the  temperature  of  the  mouth  is  98.6  F. 

8.  The  feeling  of  cold  or  warmth  not  a  true  test  of  the 
body  temperature.    It  is  well  at  this  point  to  warn  the  stu- 
dent against  confusing  the  body  temperature  with  sensations 
of  cold  or  warmth.    Just  as  visual  sensations  are  aroused 
only  by  that  light  which  falls  upon  the  sense  organ  espe- 
cially adapted  to  respond  to  its  stimulation,  namely  the  eye, 
while  light  falling  upon  the  skin  arouses  no  such  sensation, 
so  heat  and  cold  can   excite  the   corresponding  sensations 
only  when  they  act  on  special  end  organs  adapted  to  receive 
these  stimuli,  and  these  end  organs  are  found  only  in  the 
skin,  the  mouth,  and  perhaps  the  nose,  pharynx,  and  upper 
oesophagus.    We  are  therefore  conscious  only  of  the  tempera- 
ture of  these  organs ;  we  are  not  and  cannot  be  conscious  of 
the  temperature  of  the  blood  or  of  internal  organs  generally. 
It  is  therefore  clear  that  our  feelings  give  us  no  reliable  in- 
formation as  to  the  temperature  of  the  internal  parts  of  the 


194  THE  HUMAN  MECHANISM 

body.  This  fact  is  strikingly  illustrated  in  the  case  of  a  "chill," 
when  the  internal  temperature  is  almost  always  really  above, 
and  not  below,  the  normal,  and  the  feeling  of  warmth  pro- 
duced by  muscular  activity  or  by  warming  one's  self  at  a  fire 
merely  indicates  a  higher  temperature  of  the  skin,  not  a  higher 
temperature  of  internal  organs. 

Having  now  learned  the  more  obvious  facts  about  the 
constant  temperature  of  the  body,  we  have  next  to  inquire 
by  what  means  this  constant  temperature  is  maintained. 

9.  The  production  and  the  loss  of  heat.  We  must  first 
remember  that  the  body  produces  or  liberates  heat.  The 
chemical  changes,  largely  oxidative  in  character,  which  are 
at  the  basis  of  the  work  of  its  muscles,  glands,  nerve  cells, 
etc.,  liberate  heat  just  as  truly  as  the  burning  of  coal  in  the 
furnace  of  an  engine  liberates  heat.  Heat  production  is  there- 
fore an  indispensable  result  of  cellular  and  organic  activity, 
and  it  is  greatest  in  those  organs,  like  the  muscles  and  liver, 
which  carry  out  the  most  active  chemical  processes.  The 
body  is  warm  for  the  same  reason  that  a  stove  is  warm; 
that  is,  because  heat-producing  chemical  changes,  largely  of 
an  oxidative  character,  are  going  on  within  it.  In  the  second 
place,  the  body  is  always  losing  heat,  and  this  in  two  ways: 
(1)  by  the  transfer  of  heat  by  conduction,  convection,  and 
radiation  *  to  colder  objects  or  to  the  colder  air  with  which 
the  body  is  surrounded,  and  (2)  by  the  evaporation  of  water 
from  the  surfaces  of  the  body  —  especially  by  the  evaporation 
of  water  of  perspiration. 

Everyone  knows  in  a  general  way  that  when  a  warm 
body  is  brought  near  a  colder  one,  the  former  becomes  colder 
and  the  latter  warmer;  heat  is  transferred  from  the  warmer 
body  to  the  colder.  In  this  way  the  clothing  is  warmed  by 

1  Those  not  familiar  with  the  meaning  of  the  terms  "conduction,"  "con- 
vection," and  "radiation"  will  find  them  explained  in  section  26  of  this 
chapter  (p.  211).  In  the  following  discussion  we  have  arbitrarily  adopted 
the  term  "heat  transfer"  to  include  these  three  means  of  heat  loss,  in  order 
to  distinguish  them  from  the  loss  of  heat  by  evaporation. 


THEKMAL  PHENOMENA  OF  THE  BODY         195 

contact  with  the  body;  so  is  the  air  in  immediate  contact 
with  the  skin ;  and  conversely  the  body  may  be  warmed  by 
contact  with  anything  warmer  than  itself,  a  hot-water  bottle, 
for  example.  It  is  not,  however,  necessary  that  two  solid 
bodies  be  in  actual  contact  in  order  that  heat  may  pass  from 
one  to  the  other.  A  stove  warms  all  the  objects  in  a  room, 
although  few  of  them  are  touching  it;  and  the  human  body 
may  lose  heat  to,  or  gain  heat  from,  objects  at  a  greater  or 
less  distance.  The  heating  of  the  body  by  the  sun,  millions 
of  miles  away,  clearly  shows  this  fact. 

The  loss  of  heat  by  evaporation  of  water  or  other  liquid 
from  the  skin  may  be  readily  illustrated  by  the  simple  experi- 
ment of  blowing  a  gentle  current  of  cool,  dry  air  over  the 
dry  hand  and  comparing  the  cooling  thus  produced  with 
that  which  results  from  blowing  a  similar  current  against 
the  moistened  hand.  In  the  latter  case  the  cooling  will  be 
much  greater  than  in  the  former.  Liquids,  like  ether,  which 
evaporate  more  rapidly  than  water  will  produce  even  greater 
feeling  of  cold  on  the  skin. 

10.  The  heat  account  of  the  body.  The  body  is  therefore 
constantly  receiving  and  constantly  giving  out  heat,  just  as 
a  bank  is  constantly  receiving  and  paying  out  cash.  In  the 
bank  a  cash  account  is  kept,  on  one  side  of  which  is  entered 
the  cash  received  and  on  the  other  the  cash  paid  out.  The 
difference  between  the  two  sides,  known  in  business  as  the 
balance  of  the  account,  shows  how  much  cash  is  on  hand  at 
the  time  of  taking  the  balance.  Should  the  cash  unduly 
accumulate,  efforts  are  made  to  keep  down  the  balance  by 
increasing  loans;  should  the  cash  on  hand  fall  below  a 
desired  level,  active  efforts  to  encourage  loans  are  lessened 
and  the  normal  desired  balance  is  restored;  finally,  should 
there  be  an  unusual  demand  for  cash  at  the  window  of  the 
paying  teller,  for  example,  a  "  run  on  the  bank,"  the  bank 
will  borrow  from  other  banks  and  in  this  way  keep  income 
and  outgo  of  cash  approximately  equal. 


196  THE  HUMAN  MECHANISM 

In  what  follows  the  student  will  learn  that  this  is  precisely 
what  the  body  is  doing  with  regard  to  heat.  We  may,  in- 
deed, imagine  a  heat  account  of  the  body,  the  two  sides  of 
which  would  be  as  follows: 

DEBIT  CREDIT 

(Heat  received)  (Output  of  heat) 

1.  Heat  produced  within  the  body.  1.  Heat  transferred  to  surround- 

2.  Heat   transferred  to  the  body  ing  objects  colder  than  the 

from  warmer  objects  without  body  (by  conduction,  convec- 

(by  conduction,   convection,  tion,  and  radiation). 

and  radiation).  2.  Heat  lost  in  evaporating  water 

of  perspiration,  etc. 

The  balance  of  this  heat  account  at  any  one  time  is  the 
amount  of  heat  in  the  body,  and  this  determines  the  temper- 
ature of  the  body.  When  the  output  of  heat  exactly  equals 
the  heat  received,  the  balance  of  the  account  remains  the 
same;  that  is  to  say,  the  temperature  is  constant.  A  con- 
stant temperature,  therefore,  means  that  the  two  sides  of 
the  heat  account  are  being  kept  equal  to  each  other.  If 
the  balance  increases,  either  by  the  production  of  more  heat 
or  by  the  loss  of  less,  the  temperature  of  the  body  rises, 
and  we  have  fever. 

11.  Transfer  of  heat  dependent  upon  the  nature  of  the 
vehicle  of  transfer.  The  rate  at  which  heat  may  be  trans- 
ferred depends  upon  the  nature  of  the  substance  through 
which  the  transfer  occurs  and  which  we  may  speak  of  as  the 
vehicle  of  transfer.  We  cannot  go  minutely  into  the  factors 
here  concerned,  but  would  call  attention  to  the  following 
points,  which  will  be  readily  verified  from  experience: 

1.  A  gas  is  in  general  a  poorer  vehicle  of  heat  transfer  than 
a  liquid  or  a  solid.  We  make  use  of  this  fact  in  the  manu- 
facture of  fabrics  for  our  warmer  clothing,  for  these  fabrics 
are  warm  according  to  the  quantity  of  air  within  their  meshes. 
A  woolen  garment  is  warmer  than  a  cotton  garment  because 
it  contains  within  the  fabric  so  large  a  quantity  of  the  poorly 


THERMAL  PHENOMENA  OF  THE  BODY    197 

conducting  air;  or,  of  two  woolen  garments  of  the  same 
thickness,  one  of  which  is  rather  loosely  and  the  other  tightly 
woven,  the  loosely  woven  garment  will  be  much  the  warmer 
because  so  large  a  proportion  of  its  thickness  consists  of  the 
poorly  conducting  air  rather  than  of  the  rather  rapidly 
conducting  solid  woolen  fibers  (see  p.  423) „  Or,  again, 
air  of  70°  F.  is  very  comfortable ;  it  feels  neither  cold  nor 
warm  to  the  skin ;  but  water  of  70°  F0  feels  distinctly  cool. 
This  is  because  heat  is  conducted  away  from  the  skin  more 
rapidly  by  water  than  by  air.  For  this  reason  we  may  feel 
chilly  when  our  clothing  has  become  drenched  with  rain. 

2.  Moist  air  is  a  better  vehicle  of  heat  transfer  than  dry  air. 
This  becomes  obvious  when  one  is  exposed  to  damp  air  at  a 
temperature  of  less  than  70°,  and  the  familiar  difference  be- 
tween dry  and  damp  winds  in  winter  illustrates  the  same  fact, 
for  a  damp  wind  at  50°  F.  chills  the  skin  more  than  a  dry 
wind  at  40°  F.  The  student  is  cautioned,  however,  against 
supposing  that  dampness  always  favors  the  output  of  heat 
from  the  body;  it  favors  only  one  method  of  heat  output, 
namely  the  transfer  of  heat.  On  the  other  hand,  dampness 
hinders  the  output  of  heat  by  evaporation.  Hence  at  those 
temperatures  (above  80°)  where  the  output  is  chiefly  by 
evaporation,  a  damp  atmosphere  is  close,  warm,  and  muggy; 
where  the  output  is  chiefly  by  heat  transfer  (below  70°),  a 
damp  atmosphere  is  chilly. 

12.  The  evaporation  and  not  the  secretion  of  perspiration 
cools  the  body.  The  student  should  understand  clearly  that 
it  is  the  evaporation  of  the  perspiration,  not  the  secretion  of 
it,  which  abstracts  heat  from  the  body.  Perspiration  may  be 
secreted  in  large  quantities,  but  if  it  does  not  evaporate,  — 
as  happens  on  a  very  moist,  humid,  muggy  day,  when  the 
atmosphere  already  contains  about  as  much  aqueous  vapor  as 
it  can  hold,  —  it  takes  little  or  no  heat  from  the  skin.  Nor 
is  the  efficiency  of  the  perspiration  as  a  cooling  agent  meas- 
ured by  the  amount  of  visible  or  "  sensible  "  perspiration,  for 


198  THE  HUMAN  MECHANISM 

this  is  only  the  perspiration  which  has  not  evaporated ;  the 
true  measure  of  the  cooling  effect  would  be  the  perspiration 
which  has  evaporated  and  of  which  we  are  not  conscious. 

It  is  important  to  note  that  the  evaporation  of  perspira- 
tion (or  of  water  from  the  lungs  and  air  passages)  is  the 
only  means  of  cooling  the  body  when  objects  around  it  are 
warmer  than  the  body  itself.  In  this  case  the  agents  of  heat 
transfer  only  add  heat  to  the  body,  but  even  their  combined 
action  may  often  be  overcome  by  an  abundant  evaporation 
of  perspiration.  Men  have  remained  for  some  time  in  rooms 
whose  temperature  was  as  high  as  260°  F.,  or  48°  above  the 
boiling  point  of  water,  without  any  marked  rise  of  the  body 
temperature  and  without  severe  discomfort,  the  temperature 
of  the  body  being  kept  down  solely  by  the  evaporation  of 
perspiration  from  the  skin.  In  order  to  make  this  means  of 
cooling  possible,  it  is  absolutely  essential  that  the  air  be  dry 
and  capable  of  taking  up  moisture.  No  one  can  survive  long 
at  such  temperatures  in  moist  air. 

13.  The  effect  of  stagnant  versus  moving  air ;  the  aerial 
blanket.  On  a  perfectly  still  day  the  layer  of  air  about  the 
body  becomes  warmed  by  the  skin  and,  so  long  as  it  is  not 
removed,  forms  an  air-blanket  which  goes  far  to  keep  the 
skin  warm;  for  air  is  a  poor  conductor  of  heat.  As  soon, 
however,  as  a  breeze  springs  up,  convection  comes  into  play 
and  the  skin  is  cooled  more  rapidly.  In  stagnant  air,  more- 
over, the  evaporation  of  the  perspiration  tends  to  saturate 
this  air-blanket  with  water  vapor,  so  that  further  evaporation 
is  rendered  difficult.  Accordingly,  when  perspiration  is  not 
being  secreted,  moving  air  cools  the  body  by  increasing  con- 
vection ;  and  when  the  skin  is  moist  it  cools  the  body  both 
by  increasing  convection  and  by  facilitating  the  evaporation 
of  perspiration.  The  breeze  which  in  winter  is  an  unwhole- 
some draft,  in  summer  is  often  absolutely  essential  to  working 
power  as  well  as  to  bodily  comfort,  for  without  it  we  are 
clothed  in  this  aerial  blanket. 


THERMAL  PHENOMENA  OF  THE  BODY        199 

B.  THE  REGULATION  OF  THE  BODY  TEMPERATURE 

14.  How  the  balance  of  the  heat  account  may  be  disturbed. 

Events  both  within  the  body  and  in  its  immediate  surround- 
ings tend  to  change  the  balance  of  the  heat  account ;  that  is, 
to  upset  the  equilibrium  previously  existing  between  heat 
loss  and  heat  production.  The  most  important  of  these 
events  are  (1)  muscular  activity  and  the  digestion  of  food 
within,  and  (2)  changes  of  atmospheric  or  weather  condi- 
tions without.  Let  us  consider  how  each  of  these  acts. 

Muscular  activity,  by  producing  more  heat  within  the  body, 
would  tend  to  increase  the  heat  balance  ;  and,  unless  measures 
were  taken  at  the  same  time  to  increase  heat  output,  the  tem- 
perature of  the  body  would  rise.  Muscular  activity  may  double 
or  even  treble  the  heat  produced.  The  digestion  of  a  meal 
similarly  liberates  heat  within  the  body  and  so  tends  to  raise 
its  temperature,  but  the  heat  produced  in  this  case  is  far  less 
in  amount  than  that  produced  during  muscular  activity. 

Changes  of  atmospheric  or  weather  conditions  act  by  changing 
the  ease  with  which  heat  is  lost ;  and,  remembering  that  heat 
is  lost  in  two  ways, — by  transfer  to  colder  surroundings  and 
by  evaporation  of  perspiration, — we  must  inquire  how  various 
weather  conditions  influence  each  of  these  agents  of  heat  out- 
put. The  three  main  weather  conditions  are  the  temperature, 
movement,  and  moisture  of  the  air,  and  the  following  tabular 
form  will  aid  in  understanding  the  relation  of  each  of  these 
conditions  to  the  heat  output  of  the  body. 

I.  TEMPERATURE  OF  AIR 

A.  INFLUENCE  ON  HEAT  B.  INFLUENCE  ON  EVAPO- 

TRANSFER  RATION 

Heat  is  transferred  more  rapidly  The  warmer  the  air,  the  more 

to  colder  surroundings  than  to  water  vapor  it  can  take  up.  This 
surroundings  which  are  near  the  facilitates  the  evaporation  of  per- 
temperature  of  the  body.  spiration  on  a  warm  day,  when 

this  is  most  needed  to  cool  the  body. 


200 


THE  HUMAN  MECHANISM 


II.  MOVEMENT  OF  AIR 


A.  INFLUENCE  ON  HEAT 
TRANSFER 

Movement  of  air  increases  heat 
transfer  to  the  atmosphere  by  re- 
placing the  "  aerial  blanket  "  of 
warmed  air  with  colder  air,  to 
which  heat  is  transferred  more 
rapidly. 


B.  INFLUENCE  ON  EVAPO- 
RATION 

When  perspiration  is  evapo- 
rating into  stagnant  air  in  contact 
with  the  skin,  this  air  becomes 
more  nearly  saturated  with  water 
vapor,  and  its  power  of  absorbing 
water  vapor  is  lessened.  By  re- 
placing the  "aerial  blanket"  of 
muggy  air  with  dry  air,  the  out- 
put of  heat  by  evaporation  is 
greatly  favored. 


III.   HUMIDITY  OF  THE  ATMOSPHERE 


A.  INFLUENCE  ON  HEAT 
TRANSFER 

Humidity  increases  the  rate  of 
transfer  of  heat,  as  explained  on 
page  197.  This  is  of  little  impor- 
tance on  warm  days,  because  little 
heat  is  then  transferred  either  by 
dry  or  by  moist  air.  On  cooler 
days  it  is  of  great  importance. 


B.  INFLUENCE  ON  EVAPO- 
RATION 

Humidity  diminishes  the  out- 
put of  heat  by  evaporation,  because 
the  water  vapor  which  the  atmos- 
phere can  take  up  is  limited  and 
a  humid  atmosphere  is  one  already 
largely  saturated.  This  influence 
of  humidity  is  of  no  consequence 
unless  perspiration  is  being  se- 
creted, but  it  is  a  very  important 
matter  on  warm  days. 


15.  How  the  heat  balance  when  disturbed  is  restored  by 
the  body.  In  these  ways  changes  in  the  activities  of  daily 
life  and  changes  of  weather  tend  to  change  the  heat  balance 
of  the  body  —  that  is  to  say,  they  tend  to  change  the  tem- 
perature of  the  body.  And  they  would  do  this,  did  not  the 
body  possess  the  power,  within  certain  limits,  of  changing 
both  its  rate  of  heat  loss  and  its  rate  of  heat  production. 

The  rate  of  heat  loss  may  be  changed  in  two  ways: 
(1)  by  changing  the  quantity  of  blood  flowing  through  the 
skin.  Obviously  the  more  the  warmed  blood  is  kept  within 


THERMAL  PHENOMENA  OF  THE  BODY         201 

the  internal  organs,  the  smaller  will  be  the  amount  of  heat 
transferred  from  the  surface  of  the  body  to  surrounding 
objects.  The  student  now  understands  the  reason  for  the 
reactions  of  the  circulation  to  changes  of  surrounding  tem- 
perature. The  entire  vasomotor  mechanism  with  its  vaso- 
constrictor and  vasodilator  nerves  thus  forms  part  of  the 
mechanism  of  temperature  regulation.  The  rate  of  heat  loss 
may  also  be  changed  (2)  by  producing  a  secretion  of  per- 
spiration. This  secretion  begins  at  about  68°  or  70°  F.  in 
the  body  at  rest  and  increases  in  amount  as  the  external 
temperature  rises.  The  sweat  glands  are  thrown  into  action 
by  nervous  impulses.  Hence  the  nervous  system  through  its 
nerves  to  the  arterioles  and  the  sweat  glands  controls  the 
output  of  heat  from  the  body. 

The  nervous  system  also  controls  the  rate  of  heat  produc- 
tion, for  this  is  changed  by  increasing  or  diminishing  the 
activity  of  the  skeletal  muscles.  We  are  more  active  on  cold 
than  on  warm  days,  and  this  apart  from  any  conscious  adjust- 
ment of  muscular  activity  to  the  temperature  needs  of  the 
body.  We  shall  return  to  several  interesting  features  of  this 
part  of  our  subject  in  later  paragraphs. 

16.  Reactions  of  the  body  at  rest  and  lightly  clad  to  changes 
of  external  temperature.  Having  learned  the  more  important 
principles  concerned  in  maintaining  the  constant  heat  balance, 
let  us  now  observe  the  actual  behavior  of  the  body  as  the 
external  temperature  changes,  assuming  that  the  air  remains 
of  moderate  humidity  and  that  there  is  little  or  no  wind.1 
To  do  this  let  us  suppose  that  the  body  at  rest  and  lightly 
clad  is  exposed,  to  begin  with,  to  a  temperature  of  90°  F. 
At  this  point  but  little  heat  is  transferred  by  conduction,  con- 
vection, and  radiation  from  the  skin  to  surrounding  objects, 
since  both  are  so  nearly  of  the  same  temperature.  Hence  the 
main  reliance  for  getting  rid  of  the  heat  constantly  being 
liberated  is  upon  the  evaporation  of  the  perspiration,  which 
1  Consult  Fig.  87  when  reading  this  section. 


202 


THE  HUMAN  MECHANISM 


is  abundantly  secreted ;  the  cutaneous  arterioles  are  also 
widely  dilated.  Let  us  now  suppose  the  day  becomes  cooler 
and  the  temperature  falls  to  80°  F.  Heat  production  remains 
unchanged;  but  more  heat  is  now  transferred  to  the  cooler 
surrounding  objects,  and  less  is  lost  by  evaporation  because 
less  perspiration  is  secreted.  As  the  external  temperature 
falls  further,  still  more  heat  is  transferred  to  colder  objects 


o 

4 

g 

N 

to 

)l 

1 

j 

4 

ANGER 

p 

C 

I 

c 

ft 

— 

I 

• 

_^— 

—  —  ~^ 

i 
l 

1 

] 

1 

1 

1 

1 

! 

i 

i 

• 

100°  90°   80°  7fi°    72°  70°. 68°   64°    60°   50°  40°    30°    20°  10°    0°  -10* 

Heat  loss 

checked 

solely  by 

vasomotor 


Problem  is  to  get 
rid  of  the  heat 


means 
Heat  productior 


Problem  is  to  produce  heat 

enough  to  compensate  for 

the  rapid  loss 


——————*  =  Heat  lost  by  transfer  (conduction,  convection,  radiation) 

................  •ss:  Heat  lost  by  evaporation  of  perspiration 

FIG.  87.   Production  and  output  of  heat  at  different  temperatures 

and  correspondingly  less  is  lost  by  evaporation  of  the  per- 
spiration until,  somewhere  about  68°  to  70°  F.,  exactly  the 
same  amount  of  heat  is  lost  by  conduction,  convection,  and 
radiation  as  is  produced.  At  this  point  the  secretion  of  the 
perspiration  ceases. 

Thus  far  the  difficulty  in  maintaining  a  constant  tempera- 
ture has  been  that  of  getting  rid  of  heat  under  atmospheric 
conditions  which  are  unfavorable  for  the  ready  conduction, 
convection,  and  radiation  of  heat  from  the  skin.  Blood  is 


THERMAL  PHENOMENA  OF  THE  BODY    203 

brought  in  large  quantities  to  the  skin  and  correspondingly 
drawn  away  from  internal  organs,  and  the  evaporation  of 
perspiration  becomes  increasingly  important  as  the  external 
temperature  rises  from  70°  F.  to  90°  and  100°  F.  The 
organism  is  striving  against  a  rise  of  its  body  temperature. 

About  68°  or  70°  F.,  however,  the  situation  changes ;  for, 
as  the  external  temperature  continues  to  fall,  heat  begins  to 
be  transferred  to  surrounding  objects  more  rapidly  than  it  is 
produced.  The  temperature  of  the  body  would  fall  if  no 
means  were  taken  to  prevent  the  result.  Even  during  the 
fall  from  90°  to  70°  the  cutaneous  arterioles,  widely  dilated 
at  the  higher  temperature,  have  been  gradually  increasing 
their  tone  and  so  sending  diminishing  quantities  of  blood 
through  the  skin.  Below  68°  to  70°  this  tone  rapidly  increases  ; 
the  veins  are  no  longer  conspicuous  on  the  hand  and  arm; 
if  the  blood  is  forced  out  of  a  portion  of  the  skin  by  gentle 
compression  with  the  finger,  the  color  returns  slowly,  indi- 
cating considerable  constriction  of  the  cutaneous  arterioles. 
At  the  same  time  the  arterioles  of  internal  organs  are  dilat- 
ing (see  p.  152)  so  that  the  liver,  the  kidneys,  the  mucous 
membranes  of  the  alimentary  canal  and  of  the  air  passages 
contain  an  increasing  quantity  of  blood.  The  body  is  now 
striving  against  a  fall  of  its  internal  temperature  by  driving  the 
blood  from  the  skin  back  upon  internal  organs. 

By  the  time  the  temperature  has  fallen  to  60°  F.,  or  there- 
abouts, the  cutaneous  arterioles  have  constricted  to  their 
utmost,  the  blood  flow  through  the  skin  has  nearly  ceased, 
and  the  organism  has  no  means  at  command  by  which  to 
restrict  the  further  output  of  heat.  If  in  this  emergency 
heat  production  were  to  remain  constant  while  external  tem- 
perature continued  to  fall,  the  temperature  of  the  body  would 
be  lowered,  for  the  transfer  of  heat  would  not  only  continue 
but  increase.  That  it  is  not  usually  lowered  is  due  solely 
to  the  fact  that  more  heat  is  then  produced  within  the  body ; 
the  oxidations  (and  hence  heat  production)  which  have 


204  THE  HUMAN  MECHANISM 

remained  fairly  constant  in  amount  between  90°  F.  and  65°  F. 
now  increase  to  compensate  the  inevitable  loss,  and  continue  to 
increase  as  the  atmospheric  temperature  continues  to  fall.  The 
body  is  now  striving  against  the  effects  of  a  rapid  and  inevitable 
loss  of  heat  by  producing  more  heat,  and  continues  to  do  so  until 
somewhere  near  the  freezing  point  (32°  F.)  it  can  no  longer  pro- 
duce enough  heat  to  balance  the  loss ;  the  temperature  of  the 
body  then  falls  and  the  man  ultimately  freezes  to  death.1 

Briefly,  then,  at  an  external  temperature  somewhere  be- 
tween 65°  and  70°  heat  production  exactly  equals  heat  trans- 
fer, and  it  is  not  necessary  that  the  body  make  any  special 
effort  to  get  rid  of  heat  or  to  compensate  for  heat  loss. 
The  blood  is  properly  distributed  between  the  skin  and  internal 
organs,  and  there  is  no  excess  in  either.  This  we  may  call  the 
ideal  or  optimum  temperature,  for  the  given  conditions. 
Above  this  point  measures  must  be  taken  to  provide  for  an 
adequate  heat  output  by  sending  a  larger  quantity  of  blood 
to  the  skin  and  by  the  secretion  of  perspiration ;  below  this 
point  measures  of  the  opposite  kind  must  be  taken  to  check 
heat  loss  or  even  to  increase  heat  production. 

17.  Changes  of  the  optimum  temperature  with  high  humid- 
ity, with  wind,  and  with  muscular  activity.  High  humidity, 
by  facilitating  the  transfer  of  heat  from  the  body,  raises  the 
optimum  temperature  a  few  degrees;  a  room  is  comfortable 
at  65°  when  the  air  is  dry ;  it  is  too  cool  when  the  air  is 
moist.  Wind  may  raise  the  optimum  temperature  still  more, 
and  for  the  same  reason;  it  may  be  safe  to  sit  in  a  breeze 
at  75°  when  it  is  decidedly  unsafe  to  do  so  at  65°  or  70°. 

Muscular  activity  on  the  other  hand,  because  of  the  produc- 
tion of  larger  quantities  of  heat,  lowers  the  optimum  tem- 
perature, for  at  the  lower  temperature  the  agencies  of  heat 
transfer  can  get  rid  of  the  excess  of  heat  without  a  large 
blood  flow  to  the  skin  and  without  inducing  perspiration. 

1  In  all  this  it  must  be  remembered  that  the  body  is  still  lightly  clad  and 
at  rest. 


THERMAL  PHENOMENA  OF  THE  BODY 


205 


In  all  cases,  —  rest  or  muscular  activity,  high  or  low 
humidity,  wind  or  calm,  —  wherever  the  point  of  optimum 
external  temperature  may  be,  we  always  find  above  this 
point  the  region  of  active  measures  for  heat  dissipation, 
and  below  it  the  region  of  active  heat  production.  This  is 
graphically  shown  in  Fig.  88. 

18.  The  "danger  zone "  of  atmospheric  temperature.  We  have 
seen  that,  as  the  temperature  falls  from  70°  to  60°,  the  main 
agency  employed  for  temperature  regulation  is  the  diminution 
of  the  blood  flow  through  the  skin,  with  its  compensating  in- 
crease of  the,  blood  flow  within  internal  organs,  thereby  retaining 


100°      90°       80°       70°       60°       50'       40° 


Rest,  normal  humidity, 

no  wind 
Rest,  high  humidity 

Rest,  wind 
Muscular  activity 


I  =  Optimum  temperature 

I  =  Point  at  which  increased  heat  production  begins 

Blank  space  between  the  two  indicates  region  of  the  "  danger  zone" 

FIG.  88.    Variations  in  the  optimum  temperature 

as  far  as  possible  the  heat  within  the  body.  This  threatens 
serious  congestions  and  other  unhealthful  conditions,  which 
we  shall  consider  at  length  in  our  study  of  hygiene  (see 
Chap.  XXI).  It  is  because  the  temperature  of  a  room  may 
fall  from  66°  to  60°  so  gradually  that  we  do  not  notice  it 
until  the  internal  damage  is  done,  whereas  it  could  not  fall 
to  50°  or  40°  without  our  noticing  it  and  correcting  the 
trouble,  that  more  colds  are  taken  in  the  former  case  than 
in  the  latter.  In  other  words,  as  the  temperature  goes  below 
65°  the  body  seems  at  first  to  rely  wholly  on  the  vascular 
mechanism  of  temperature  regulation,  and  does  not  begin  to 
produce  more  heat  until  this  resource  has  been  utilized  not 
only  to  its  utmost,  but  even  to  an  extent  inconsistent  with 
health.  The  "danger  zone"  temperature  may  then  be  defined 


206  THE  HUMAN  MECHANISM 

as  beginning  a  degree  or  two  below  the  ideal  or  optimum 
temperature  and  extending  about  five  degrees  below  this 
point.  Like  the  optimum  temperature,  its  exact  position 
varies  with  atmospheric  conditions  and  with  the  amount 
of  muscular  activity. 

19.  The  influence  of  clothing.    In  the  discussion  above  we 
have  assumed  that  the  clothing  has  not  been  changed  with 
the  change  of  external  temperature,  etc.    Clothing,  however, 
may  modify  greatly  the  figures  given  above,  for  it  interferes 
with  the  loss  of  heat  from  the  skin,  and  the  obvious  effect 
of  increasing  its  weight  is  to  lower  the  optimum  temperature 
and  the  region  of   dangerous  temperature.     By  changes   of 
clothing,  by  muscular  activity,  and  by  the  use  of  fans,  man 
has  it  in   his   power  to   supplement  the  unconscious  reflex 
adjustments  which  we  have  thus  far  been  studying  by  a 
conscious   adaptation  to  changing  conditions   of  climate  or 
weather.    The  hygienic  use   of   clothing   will  be   discussed 
in  Chapter  XXVI. 

20.  Temperature   regulation   and  muscular   activity.    The 
reactions  of  the  body  to  maintain  its  constant  temperature 
during  muscular  activity  are  familiar  to   everyone,  and  it 
is  only  necessary  to  sum  them  up  and  to  point  out  some 
practical  applications.    The  arterioles  of  the  skin  are  dilated 
(while  those  of  internal  organs  are  constricted)  and  perspi- 
ration is  secreted.     These  are  the  same  reactions  which  are 
noticed  when  the  body  is  exposed  to  external  warmth,  and 
their  purpose  is  the  same  in  both  cases  —  to  facilitate  the 
escape  of  heat.    But  in  the  one  case  they  are  made  necessary 
by  the  fact  that  climatic  conditions  interfere  with  the  out- 
put of  heat,  in  the  other  by  the  fact  that  more  heat  is  being 
liberated  and  hence  more  must  be  got  rid  of. 

Seldom  indeed  is  so  severe  a  strain  imposed  upon  the 
mechanism  of  heat  dissipation  as  during  vigorous  muscular 
exertion,  and  especially  when  the  external  conditions  are  not 
favorable  for  the  output  of  heat.  Caution  is  then  urgently 


THERMAL  PHENOMENA  OF  THE  BODY        207 

indicated  lest  we  make  the  strain  too  great.  It  is  a  practical 
point  to  remember  in  this  connection  that  some  forms  of 
muscular  exertion  introduce  conditions  for  getting  rid  of  the 
surplus  heat  much  more  readily  than  others;  this  is  especially 
true  of  those  which  involve  movement  of  the  body  as  a 
whole.  Bicycle  or  horseback  riding,  by  creating  a  •  breeze, 
renders  the  cooling  of  the  body  a  much  easier  matter  than 
does  sawing  wood,  or  swinging  Indian  clubs,  or  gymnastic 
work  in  general;  again,  a  particular  form  of  exercise  on  a 
dry  day,  when  the  perspiration  can  evaporate  readily,  may 
be  safe,  while  it  would  be  decidedly  inadvisable  on  a  muggy 
day,  even  though  the  temperature  were  somewhat  lower. 
Indeed,  by  this  time  the  student  must  have  learned  that 
the  thermometer  alone  is  no  safe  indicator  of  the  difficulty 
of  heat  elimination  in  warm  weather. 

21.  Relations  of  climatic   conditions  to  mental   work  and 
sleep.    During  mental  work  the  brain  requires  an  increased 
supply  of  blood,  and  this  is  obtained  partly  by  diminishing 
the  supply  to  the  skin  (constriction  of  cutaneous  arteries) ; 
during  sleep,  on  the  other  hand,  the  supply  to  the  brain  is 
diminished,   and  this   is  ordinarily  effected  by   dilating  the 
arteries  of  the  skin  (see  p.  155).    Mental  work  is  difficult 
on   very  warm   days,  partly  because  it  is  difficult  to  bring 
about  cutaneous  constriction ;    and   it  is   especially  difficult 
on  warm,  muggy  days,  since  the  maintenance  of  the  constant 
temperature  then  requires  an  excessive  cutaneous  dilation,  and 
the  brain  is  quite  unable  to  command  its  needed  blood  supply. 

It  is  also  clear  that  since  the  arterioles  of  the  skin  should 
dilate  during  sleep,  and  since  they  cannot  readily  do  this 
when  the  skin  is  exposed  to  cold,  to  "  sleep  warm  "  is  good 
advice,  based  on  sound  physiological  principles. 

22.  Digestion  and  the  maintenance  of  the  constant  temper- 
ature.    During  digestion,  and  especially  during  its    earlier 
stages,  when  secretion  is   at  its  maximum,   a  large  supply 
of  blood  is  needed  in  the  stomach,  the  pancreas,  and  the 


208  THE  HUMAN  MECHANISM 

intestine.  This  cannot  readily  be  secured  when  blood  is 
being  sent  in  large  quantities  to  the  skin  in  order  to  cool 
the  body.  We  have  seen  all  along  that  the  two  great  vas- 
cular areas  of  the  skin  and  digestive  organs  are  more  or  less 
antagonistic  or  compensating  in  their  vasomotor  reactions. 
When  the  blood  is  present  in  large  quantities  in  the  skin, 
it  is  present  in  smaller  quantities  in  the  stomach,  the  intes- 
tine, the  pancreas,  the  liver;  and,  vice  versa,  these  organs 
can  best  obtain  an  adequate  blood  supply  when  the,  de- 
mands of  the  skin  are  not  excessive.  Consequently  diges- 
tion is  more  difficult  in  warm  than  in  cold  weather,  and  we 
should  then  eat  less  at  a  time,  even  if  we  have  to  eat 
somewhat  more  frequently. 

During  the  digestion  of  a  meal  the  chemical  activities  of 
secretion,  the  peristaltic  muscular  movements,  etc.,  somewhat 
increase  heat  production  in  the  body;  and  this  increase, 
though  not  great,  is  at  times  great  enough  to  make  us  feel 
distinctly  warmer.  When  one  is  slightly  chilly,  for  example, 
he  often  feels  warmer  after  eating  something,  even  though 
the  meal  be  cold;  and  on  a  very  warm,  muggy  day,  when 
the  blood  flow  through  the  skin  is  already  excessive  and  its 
temperature  unduly  high,  the  digestion  of  a  meal  often  adds 
to  the  discomfort,  because  the  larger  production  of  heat  leads 
to  further  dilation  of  the  skin  vessels. 

23.  The  mechanism  of  temperature  regulation.  The  pre- 
ceding pages  have  shown  us  that  temperature  regulation 
depends  chiefly  on  three  physiological  mechanisms:  (1)  the 
vasomotor  system,  which  controls  the  distribution  of  blood 
between  the  skin  and  the  internal  organs ;  (2)  the  sweat 
glands;  (3)  the  mechanism  of  heat  production.  The  first 
of  these  has  already  been  described  in  the  study  of  the 
circulation.  The  heating  of  the  skin  stimulates  afferent 
nerves  which  reflexly  dilate  the  arteries  of  the  skin  and 
also  simultaneously  constrict  those  of  internal  organs.  This 
reflex,  then,  is  dependent  on  the  temperature  of  the  skin ; 


THEEMAL  PHENOMENA  OF  THE  BODY 


209 


anything  which  heats  the  skin  causes  a  reflex  dilation  of  its 
arterioles  and  lessens  the  supply  of  blood  to  internal  organs. 
The  secretion  of  perspiration  is  also  under  the  control 
of  the  nervous  system.  The  sweat  glands,  like  the  salivary 
glands,  receive  nerves,  and  secrete  only  in  response  to  their 
stimulation.  When  the  nerves  going  to  the  sweat  glands  of 
any  region  are  injured,  exposure  of  these  glands  to  external 


Nerve  Endings  affected 
by  Warmth 


FIG.  89.   Diagram  of  the  cutaneous  reflexes  of  temperature  regulation 

Showing  the  epidermis,  blood  vessels  of  the  dermis,  a  sweat  gland,  and  the 
nervous  mechanism  governing  blood  vessels  and  sweat  glands 

warmth  produces  no  perspiration;  stimulation  of  their  nerves, 
however,  produces  a  copious  secretion. 

24.  The  skeletal  muscles  the  main  organs  in  the  regulation 
of  heat  production.  The  third  mechanism  of  heat  regulation 
is  that  whereby  the  amount  of  heat  produced  is  increased 
as  it  is  needed.  The  main  organs  here  concerned  are  the 
skeletal  muscles.  As  the  afferent  impulses  started  in  the 
skin  by  the  stimulation  of  cold  become  stronger,  they  ulti- 
mately stimulate  reflexly  the  skeletal  muscles  to  contraction, 
and  so  to  the  production  of  heat.  This  contraction  does  not 


210  THE  HUMAN  MECHANISM 

ordinarily  produce  motion,  because  antagonistic  muscles  are 
stimulated  equally;  but  in  another  way  we  are  often  con- 
scious of  this  increased  muscular  action.  Everyone  knows 
the  difference  between  the  "  bracing "  effects  of  a  cool  or 
cold  day  and  the  "  relaxed,"  "  slack-twisted "  feeling  on  a 
warm  day ;  and  this  is  largely  traceable  to  the  sensations 
which  come  from  the  contracting  muscles  in  the  former  case 
and  to  the  absence  of  such  sensations  from  the  inactive 
muscles  in  the  latter.  To  put  it  in  another  way,  cold 
increases  the  tone  of  the  skeletal  muscles  (see  p.  161).  A 
skeletal  muscle  on  a  cold  day  is  never  completely  relaxed ; 
like  the  unstriped  muscles  of  the  arteries,  it  is  in  a  con- 
dition somewhere  between  extreme  contraction  and  extreme 
relaxation. 

This  muscular  reflex  also  betrays  itself  in  shivering. 
Ordinarily  the  reflex  contraction  consists  of  an  even,  steady 
tone,  but  at  times  it  becomes  more  or  less  incoordinated, 
and  shivering  results. 

25.  The  regulation  of  the  body  temperature  a  function  of  the 
nervous  system.  We  may  close  this  brief  account  of  thermal 
phenomena  of  the  body  by  recalling  to  the  attention  of  the 
student  what  must  now  be  obvious  at  a  glance ;  namely,  that 
a  constant  temperature  is  maintained  by  the  coordinating 
action  of  very  many  nervous  reflexes.  The  action  of  the 
vasomotors  of  the  skin  and  of  the  internal  organs,  of  the 
nerves  of  the  sweat  glands  and  of  the  motor  nerves  of 
the  skeletal  muscles  must  all  be  so  adjusted  with  regard 
to  one  another  that  exactly  the  right  balance  is  preserved 
amid  all  the  variations  of  heat  production  and  of  climatic 
conditions  which  affect  heat  loss.  Success  in  this  adjustment 
depends  upon  the  skill  with  which  the  coordinating  nervous 
system  does  its  part.  With  the  single  exception  of  muscular 
exertion,  no  condition  of  life  makes  such  far-reaching  or 
such  imperious  demands  upon  the  system  as  a  whole  as  does 
the  maintenance  of  the  proper  internal  temperature.  Mental 


THERMAL  PHENOMENA  OF  THE  BODY         211 

work  and  the  efficiency  of  digestion  are  examples  we  have 
already  studied  —  and  more  could  easily  be  cited  —  of  func- 
tions which,  important  as  they  are,  are  subordinated,  even 
sacrificed,  to  prevent  a  marked  rise  or  fall  in  the  temperature 
of  the  blood. 

To  such  an  extent  is  the  nervous  system  as  a  whole 
adapted  to  maintain  the  constant  temperature,  that  the  failure 
to  do  this,  as  shown  by  the  presence  of  fever  or  by  the  even 
more  serious  subnormal  temperature,  becomes  one  of  the 
most  important  indications  that  something  has  gone  wrong. 
We  know  already  how  the  nervous  system  intervenes  in 
every  function  of  our  lives,  and  how  the  well-being  of  the 
body  as  a  whole  depends  upon  the  adjustments  which  it 
brings  about.  It  is  for  these  reasons  that,  when  it  is  no 
longer  able  to  exercise  that  firm  control  of  the  constant  tem- 
perature which  is  one  of  its  most  characteristic  features  in 
health,  the  physician's  orders  usually  are  to  "  go  to  bed  and 
be  perfectly  quiet."  The  body  is  then  in  no  condition  to 
make  demands  on  the  nervous  system  for  action ;  and  a  per- 
son who  refuses  to  heed  the  plain  warning  which  his  tempera- 
ture holds  out  has  nothing  but  his  own  foolishness  to  blame 
if  he  suffers  serious  consequences. 

26.  Definitions.  Those  not  familiar  with  the  exact  meaning 
of  the  terms  "  conduction,"  "  convection,"  and  "  radiation  " 
will  find  the  following  helpful. 

Conduction.  Whenever  heat  is  transferred  directly  from  one 
mass  of  matter  to  another  with  which  it  is  in  contact,  such 
transfer  is  known  as  conduction.  A  good  example  is  the  heating 
of  a  poker  in  a  fire ;  the  heat  of  burning  coal  is  communicated 
directly  to  the  outer  particles  of  iron  and  then  from  one  particle 
of  the  iron  to  another.  The  particles  of  iron  do  not  move  up  and 
down  the  length  of  the  poker ;  each  one  simply  passes  on  to  the 
next  the  heat  it  has  received,  and  finally  those  of  the  handle  com- 
municate their  heat  to  the  hand.  All  transfer  of  heat  along  solid 
objects,  or  from  one  mass  of  matter  to  another  with  which  it  is  in 
immediate  contact,  is  by  means  of  conduction. 


212  THE  HUMAN  MECHANISM 

Solids  and  liquids  are  much  better  conductors  of  heat  than 
gases,  and  air  when  perfectly  still  is  one  of  the  poorest  con- 
ductors of  heat  with  which  we  have  to  deal.  It  is  a  familiar 
fact  that  the  skin  is  chilled  much  more  rapidly  by  water  than 
by  air  of  the  same  temperature  (why?);  and  we  shall  learn  in 
hygiene  that  warm  fabrics  owe  their  warmth  mainly  to  the 
amount  of  poor-conducting  air  stagnant  within  their  meshes. 

Convection.  When  a  warm  body  is  surrounded  by  a  fluid  such 
as  water  or  air,  heat  is  similarly  conducted  to  the  adjacent  layer 
of  water  or  air,  which  thus  becomes  warmer ;  but,  unlike  the  case 
of  the  solid,  this  heated  layer  now  moves  off,  carrying  its  heat 
with  it  to  other  parts  of  the  gas  or  liquid,  and  so  communicating 
it  to  other  matter  with  which  it  subsequently  comes  in  contact. 
This  method  of  heat  transfer  is  known  as  convection,  which,  it 
will  be  seen,  depends  at  bottom  upon  conduction,  but  which  is  at 
the  same  time  conduction  modified  by  the  movement  of  a  heated 
gas  or  liquid.  So  long  as  the  air  around  us  is  at  rest,  it  does  not 
remove  heat  readily  from  the  skin,  since  air  is  a  poor  conductor. 
Air  in  motion,  on  the  other  hand  (as  in  fanning),  cools  the  skin 
more  rapidly,  because  as  each  part  of  the  air  is  heated,  it  is  moved 
away  and  replaced  by  colder  air.  In  this  case  the  air  cools  the 
skin  by  convection  (Latin  con,  "  with"  ;  veliere,  "to  carry"  ). 

The  transfer  of  heat  from  the  internal  heat-producing  organs 
to  the  skin  affords  an  excellent  example  of  the  difference  between 
conduction  and  convection,  for  some  of  this  heat  passes  by  direct 
conduction  through  the  subcutaneous  tissue  to  the  overlying  skin, 
while  some  of  it  is  carried  to  the  surface  by  convection  in  the  blood 
stream.  When  'the  arterioles  of  the  skin  are  dilated,  convection  is 
an  important  means  of  heat  transfer  to  the  surface ;  when,  in  the 
reverse  case,  the  cutaneous  arterioles  are  constricted  to  their 
utmost,  convection  becomes  relatively  unimportant  and  direct 
conduction  alone  remains  as  the  chief  means  of  heat  transfer  to 
the  skin.  Moreover,  when  the  subcutaneous  tissue  contains  large 
amounts  of  fat,  it  is  a  poor  conductor  of  heat,  and  for  this  reason 
fat  people  when  sitting  still  on  cold  days  often  feel  colder  than 
lean  people  do. 

Radiation.  Heat  is  thus  removed  from  the  skin  by  conduction, 
and  at  times  to  an  even  greater  extent  by  convection.  But  there 
is  still  a  third  method  of  heat  loss,  known  as  radiation,  by  which 


THERMAL  PHENOMENA  OF  THE  BODY         213 

heat  can  be  transferred  across  a  space  in  which  there  is  neither 
solid,  liquid,  nor  gas,  and  in  which  conduction  and  convection  are 
consequently  impossible.  The  most  familiar  and  striking  example 
of  radiation  is  the  transfer  of  heat  from  the  sun  to  the  earth,  since 
there  is  no  atmosphere  in  the  greater  part  of  the  more  than  ninety 
millions  of  miles  of  space  which  separate  us  from  that  intensely 
heated  body. 

Any  detailed  consideration  of  radiation  belongs  to  the  domain 
of  physics  rather  than  physiology  and  would  be  out  of  place 
here.  It  is  enough  for  our  present  purposes  to  understand  that, 
whether  a  solid  body  be  in  an  atmosphere  of  air,  or  in  a  trans- 
parent liquid,  or  even  in  a  vacuum,  it  transfers  or  loses  heat  by 
direct  radiation  to  colder  objects  about  it.  From  an  open  fire  heat 
may  be  transferred  by  conduction  to  andirons  or  walls  in  direct 
contact  with  it;  or  by  convection  through  heated  air  currents  to 
the  chimney  top ;  or,  finally,  by  radiation  to  persons  standing  in 
front  of  it.  In  the  latter  case  the  heating  is  chiefly  by  radiation, 
since  there  is  no  contact  with  the  fire,  and  such  air  currents  as 
exist  are  mostly  composed  of  cool  air  sucked  towards  and  into 
the  chimney  by  its  draft.  It  is  for  these  reasons  that  open  fires 
are  said  to  "  roast  people  in  front  and  freeze  them  behind."  Con- 
versely, the  human  body,  if  warmer  than  its  surroundings,  may 
lose  heat  by  conduction,  convection,  and  radiation  to  cooler  objects 
in  the  vicinity. 

The  practical  importance  of  these  facts  is  seldom  realized.  It 
often  happens  that  the  air  in  contact  with  the  skin  is  of  the  proper 
room  temperature ;  and  yet,  if  one  is  sitting  too  near  a  cold  wall 
or  window,  enough  heat  may  be  lost  by  radiation  from  the  skin 
to  the  cold  wall,  through  the  warm  air,  to  chill  the  skin  materially, 
causing  a  loss  of  heat  and  a  "  cold." 

Laws  of  conduction  and  radiation.  For  our  purposes  the  two 
most  important  factors  which  determine  the  loss  of  heat  by  con- 
duction and  radiation  are  (1)  the  difference  in  the  temperature  of 
the  two  objects  and  (2)  the  distance  between  them.  In  general, 
the  greater  the  difference  of  temperature,  the  more  heat  will  be 
lost  from  the  warmer  to  the  colder  object ;  thus  the  skin  loses 
heat  rapidly  by  these  means  when  surrounding  objects  are  at 
0°  F.,  but  only  slowly  when  they  are  at  90°.  It  is  also  clear  that 
as  soon  as  the  temperature  of  surrounding  objects  and  of  the 


214  THE  HUMAN  MECHANISM 

atmosphere  is  as  high  as  that  of  the  body  (98.6°  F.),  no  further 
heat  can  be  lost  by  conduction  and  radiation;  and  that  above 
98.6°  F.  heat  is  conducted  and  radiated  to  the  body,  not  from  it. 
Furthermore,  the  greater  the  distance  of  the  colder  object  from 
the  body,  the  less  heat  will  the  body  lose  to  it.  Here  heat  loss 
takes  place  inversely  as  the  square  of  the  distance ;  that  is,  when 
we  are  twice  as  far  away  from  a  cold  (closed)  window,  we  lose 
only  one  fourth  as  much  heat  through  it  by  radiation ;  if  we  are 
three  times  as  far  away,  we  lose  only  one  ninth  as  much,  and 
so  on.  Consequently  we  rapidly  diminish  radiation  from  our 
bodies  by  sitting  farther  away  from  the  walls  of  a  room ;  and  it 
is  important  to  have  our  living  rooms  large  enough  to  make  it 
unnecessary  to  sit  near  the  windows  or  near  a  cold  outer  wall 
in  very  cold  weather. 


CHAPTER  XIII 

NUTEITION 

A.  THE  SOURCES  OF  POWER  AND  HEAT  FOR  THE 
HUMAN  MECHANISM 

1 .  Food  and  nutrition.    In  general  food  must  meet   the 
following   fundamental   needs    of   the   body:    first,   it   must 
supply  power  for  the  work  of  muscles,  heart,  etc. ;    second, 
it  must  give,  through  oxidative  or  other  chemical  change, 
the  heat  necessary  to  maintain  the  body  temperature ;  third, 
it  must  supply  all  the  material  needed  for  the  manufacture 
of  everything  that  enters   into  the  structure   of  the  living 
cell  (growth  and  repair)  and  also  of  the  secretions,  internal 
and  external,  the  hormones,  and  all  other  special  compounds 
which  play  any  r61e  in  the  working  of  the  human  machine. 
Since  the  first  two  of  these  functions  are  met  by  the  same 
food  material  and  in  much  the  same  way,  we  may  consider 
first  this  aspect  of  nutrition. 

2.  The  fuel  value  of  food.    In  any  locomotive  engine  the 
same  amount  of  a  given  fuel  will  enable  the  engine  to  pull 
a  train  of  the  same  weight  for  the  same  distance  over  the 
same  track,  provided,  of  course,  the  engine  itself,  the  bear- 
ings of  the  wheels,  etc.,  are  in  the  same  condition.    When  a 
ton  of  coal  is  put  into  the  tender,  it  is  with  the  expectation 
that  it  will  move  the  train  a  certain  distance.    Thus  there  is 
a  definite  relation  between  the  fuel   burned   and  the  work 
done.    Every    engineer   knows   also   that    the    same    weight 
of   different  fuels  will  carry  the  train   different   distances; 
a  thousand   pounds   of   wood,   of  bituminous  coal,    and  of 
anthracite  coal  have  different  fuel  values. 

215 


216  THE  HUMAN  MECHANISM 

The  same  weight  of  a  given  fuel  when  burned  will  always 
yield  exactly  the  same  amount  of  heat,  as  is  proved  by 
burning  the  fuel  under  conditions  which  enable  us  to  meas- 
ure the  heat  given  off.  The  simplest  means  of  doing  this  is 
perhaps  with  the  ice  calorimeter  —  a  metal  box  within  which 
the  fuel  is  burned,  the  box  being  everywhere  surrounded 
by  a  thick  layer  of  ice.  The  heat  produced  in  burning  the 
fuel  is  measured  by  the  amount  of  ice  melted. 

In  this  way  we  may  find  the  relative  amounts  of  work 
which  can  be  done  with  two  different  fuels,  for  it  has  been 
discovered  by  actual  experiment  that  if  one  kind  of.  fuel 
produces  twice  as  much  heat  as  another,  it  will  also  do 
twice  as  much  work. 

Now  food  is  the  fuel  for  the  muscular  work  of  the  body 
and  also  for  the  liberation  of  heat.  Consequently,  if  we 
determine  how  much  heat  is  liberated  when  a  certain  amount 
of  protein,  or  fat,  or  carbohydrate  is  burned  in  a  calorimeter, 
we  know  how  much  work  it  may  do  in  the  body ;  or  at  least 
we  know  that  it  can  do  no  more  than  the  amount  indicated 
by  the  calorimetric  experiment. 

3.  Units  of  heat  and  work.  In  order  to  measure  we  must 
have  units  of  measurement.  Common  units  of  length  are  the 
inch  or  centimeter ;  units  of  area  are  the  square  yard,  the 
square  meter,  or  the  acre ;  units  of  volume,  the  quart  or 
peck ;  units  of  weight,  the  pound  or  kilogram.  We  express 
the  results  of  these  measurements  by  saying  that  a  thing  is 
so  many  inches  long  or  of  so  many  pounds  weight.  What  are 
the  units  of  heat  and  work  ? 

Like  all  units,  these  are  arbitrarily  chosen.  The  unit  of 
heat,  known  as  the  calorie,  is  the  amount  of  heat  necessary 
to  raise  one  kilogram  of  water  one  degree  Centigrade.  The 
unit  of  work  is  the  amount  of  work  done  in  lifting  a  kilogram 
(2.2  Ib.)  to  the  height  of  one  meter  (39.37  in.)  from  the 
surface  of  the  earth  against  the  attraction  of  gravitation. 
This  is  known  as  the  kilogrammeter.  Thus,  when  a  man 


NUTRITION  217 

weighing  sixty  kilograms  goes  up  a  flight  of  stairs  ten  meters 
high,  his  muscles  do  600  kilogrammeters  of  work.1 

Finally,  it  has  been  found  that  the  same  fuel  which  when 
burned  will  liberate  one  calorie  of  heat  will  supply  the  power 
to  do  423.985  kilogrammeters  of  work.  By  this  we  mean 
that  not  more  than  423.985  kilogrammeters  of  work  can  be 
obtained  from  it.  Not  every  engine  is  so  perfectly  con- 
structed as  to  get  from  a  certain  fuel  its  full  working 
capacity ;  indeed,  most  engines  transform  only  a  small  frac- 
tion of  the  power  of  their  fuel  into  work,  the  rest  escaping 
as  heat  —  in  the  smoke,  or  by  radiation,  conduction,  and 
convection  from  the  furnace,  boiler,  etc.  But  by  the  method 
above  outlined  it  is  possible  to  find  the  maximum  amount 
of  work  which  can  be  obtained  from  a  given  weight  of  fuel. 

Applying  the  same  methods  to  food,  we  find  that 

1  gram  of  dried  protein  yields  4.1  calories. 

1  gram  of  dried  carbohydrate  yields  4.1  calories. 

1  gram  of  fat  yields  9.3  calories. 

These  figures  are  known  as  the  fuel  values  of  proteins, 
carbohydrates,  and  fats. 

But  the  total  possible  power  which  may  be  obtained  by 
actually  burning  a  certain  substance  under  the  most  favor- 
able conditions  is  one  thing,  and  the  amount  of  power  which 
the  muscles  may  obtain  from  it  is  quite  another.  When  coal 
is  burned  in  an  engine  it  does  work,  but  the  human  body 
would  get  no  energy  for  its  muscular  work  from  eating  coal. 
So  that  we  have  now  to  inquire  from  what  nutrients  the 
muscles  get  their  energy  for  work  and  from  what  nutrients 
the  body  derives  its  heat. 

4.  The  power  for  muscular  work.  Few  questions  in  physi- 
ology have  been  more  thoroughly  investigated  than  this. 
In  the  first  half  of  the  nineteenth  century  many  investi- 
gators, impressed  with  the  fact  that  the  muscle  fiber  yields, 

1  Work  may  also  be  expressed  in  foot-pounds.    (How  many  foot-pounds 
equal  one  kilogrammeter  ?) 
p 


218  THE  HUMAN  MECHANISM 

on  chemical  -analysis,  large  quantities  of  protein  and  only 
traces  of  carbohydrates  and  fats,  believed  that  the  energy 
for  muscular  contraction  comes  entirely  from  the  consump- 
tion of  the  protein  of  the  muscle  substance.  If  this  were 
true,  it  would  necessarily  follow  that  protein  is  the  proper 
food  to  yield  the  energy  for  muscular  contraction,  while  fats 
and  carbohydrates  would  simply  be  oxidized  to  give  heat. 

This  view  was  disproved  by  the  following  epoch-making 
experiment  of  physiology:  Two  observers  determined  for 
three  successive  days  the  nitrogen  excreted  by  themselves; 
since  almost  all  this  nitrogen  comes  from  protein,  this  gave 
the  amount  of  protein  consumed  by  the  body.  On  the  first 
and  third  days  no  vigorous  muscular  work  was  done ;  on  the 
second  day  they  climbed  a  mountain  1956  meters  (6415  ft.) 
high.  As  one  man  weighed  66  kilograms  and  the  other 
76  kilograms,  the  work  done  in  lifting  the  body  to  the  top 
of  the  mountain  in  the  two  cases  was  129,096  and  148,656 
kilogrammeters  respectively.  The  protein  which  was  oxidized 
in  this  time  could  in  the  two  cases  have  yielded  power  for 
only  68,690  and  68,376  kilogrammeters  of  work.  In  other 
words,  the  protein  did  not  begin  to  yield  sufficient  power 
for  the  muscular  work  done  in  lifting  the  body  to  the  top 
of  the  mountain ;  something  else  than  protein  must  have 
been  oxidized  for  that  purpose,  and  that  something  must 
evidently  have  been  carbohydrate  or  fat,  or  both. 

Again,  it  was  noticed  that  there  was  no  increase  of  pro- 
tein disintegration  on  the  day  of  work;  this  remained 
practically  unaffected  by  muscular  contraction.  Numerous 
other  experiments  made  since  that  time  have  shown  the 
same  thing.  Muscular  exercise  does  not  necessarily  increase 
protein  disintegration,  and  the  power  for  it  can  be  obtained 
from  fats  and  carbohydrates. 

In  the  experiment  above  referred  to  no  determinations 
were  made  of  the  excretion  of  carbon  dioxide.  Since  then 
numerous  experiments  have  been  made  in  which,  on  an 


NUTKITION  219 

abundant  mixed  diet,  both  the  nitrogen  and  the  carbon  of 
the  excretions  were  accurately  determined.  These  have 
shown  that  while  muscular  exercise  does  not  necessarily  increase 
protein  disintegration,  it  invariably  increases  the  production  of 
carbon  dioxide.  If  the  carbon  of  the  carbon  dioxide  came 
from  protein,  it  would  be  accompanied  by  increased  excretion 
of  nitrogen  derived  from  the  broken-down  protein.  The  fact 
that  it  is  not  so  accompanied  can  only  mean  that  it  came 
from  the  oxidation  of  something  which  did  not  contain 
nitrogen,  that  is,  from  fat  or  carbohydrate,  or  both. 

But  while  muscular  work  does  not  necessarily  or  even 
usually  increase  protein  decomposition,  and  the  power  for 
the  same  may  be  derived  largely,  if  not  entirely,  from  car- 
bohydrates and  fats,  it  has  been  shown  conclusively  that 
under  certain  conditions  this  power  may  come  entirely  from 
protein.  In  one  experiment  a  large  and  very  lean  dog  was 
fed  for  several  weeks  on  an  abundant  diet  of  lean  meat, 
containing  practically  no  carbohydrate  or  fat;  during  this 
time  the  dog  did  large  amounts  of  work  in  a  treadmill  and 
in  other  ways;  and  since  it  was  found  that  this  work  could 
be  done  for  weeks  •at  a  time  on  the  meat  diet,  we  conclude 
that  the  protein  must  have  been  the  sole  source  of  power 
for  the  work ;  it  must  also  have  served  as  the  source  of 
heat  production,  for  the  normal  temperature  of  the  animal 
was  maintained. 

5.  Summary  of  considerations  on  the  supply  of  power  for 
work.  .  These  and  other  experiments  show  (1)  that  the 
animal  body  can  get  its  energy  for  mechanical  work  and  for 
the  production  of  heat  from  protein,  or  from  carbohydrate, 
or  from  fat;  (2)  that  when  the  animal  is  on  an  abundant 
mixed  diet,  even  vigorous  muscular  work  does  not  increase 
the  oxidation  of  protein,1  but  does  enormously  increase  that 

1  Under  the  abnormal  conditions  of  excessive  muscular  work  (for  ex- 
ample, six-day  walking  matches  or  bicycle  races)  the  protein  oxidation  is 
often  increased. 


220  THE  HUMAN  MECHANISM 

of  carbohydrates  and  fats.  The  probable  meaning  of  this  is 
to  be  sought  in  the  fact  that  protein  decomposition  depends 
primarily  not  on  muscular  work  but,  as  we  shall  see  later, 
on  the  amount  of  protein  eaten;  while  the  oxidation  of  fats 
and  carbohydrates  depends  almost  entirely  on  the  demands  of 
the  body  for  energy  and  is  largely  independent  of  the  amount 
of  these  foods  eaten. 

6.  The  supply  of  energy  for  heat  production;    "  heating  " 
foods.    In  studying  the  phenomena  of  heat  production  in  the 
body  we  found  that  when  the  body  needs  more  heat  in  order 
to   maintain   its   normal  temperature,  this   heat  is   supplied 
chiefly  by  greater  chemical  activity  in  the  muscles  (p.  209). 
The  contraction  or  tone  of  the  muscles  increases  in  response 
to  stimuli  from  the  same  motor  nerves  which  stimulate  them 
to  activity  when  they  do  external  mechanical  work.    Heat 
production  in  the  body,  from  the  standpoint  of  nutrition,  is 
therefore,  as  far  as  we   know,  largely  a  case  of  increased 
muscular  activity ;    and  here,  as  in  the  case  of  mechanical 
work,  the  energy  can  be  obtained  from  one  kind  of  food  as 
well  as  from  another.    Contrary  to  popular  ideas  we  have 
no  conclusive  evidence  that  one  kind  of  food  supplies  heat 
more  readily  than  another.    What  is  required  in  cold  weather 
is  more  food,  whether  protein  or  carbohydrate  or  fat.    We 
shall  see  that  there  are  good  reasons  for  not  unduly  increas- 
ing the  protein  of  the  diet  under  any  conditions,  and  hence 
in  this  special  case  it  is  probably  better  to  increase  the  non- 
nitrogenous   foods   to   a  greater   extent  than   the   proteins, 
though  not  because  they  are  better  "  heating "  foods. 

7.  The  daily  requirement  of  the  body  for  power  and  heat. 
How  many  calories  must  be  furnished  the  body  to  cover  its 
daily  needs  for  work  and  for  the  maintenance  of  its  temper- 
ature ?    This  question  has  been  studied  by  several  methods, 
but  we  must  content  ourselves  with  a  statement  of  some  of 
the  most  important  results.     Healthy  people  whose  choice 
of  food  is  not  restricted  by  financial  considerations,  but  is 


NUTKITION  221 

determined  solely  by  appetite  and  the  feeding  customs  of 
their  home  or  community,  usually  consume  each  day  food 
of  a  fuel  value  of  20  calories  per  pound  of  body  weight. 
It  is  exceptional  to  find  less  than  16  calories  or  more  than 
24  so  long  as  only  moderate  amounts  of  exercise  are  taken; 
and  many  students  of  this  subject  have  assumed  that  one 
requirement  of  diet  is  that  the  daily  intake  of  food  should 
have  approximately  this  fuel  value. 

This  view  has,  however,  been  seriously  questioned  by 
careful  and  competent  observers,  and  their  work  seems  to 
show  that  a  fuel  value  of  13.5  calories  per  pound  of  body 
weight  more  nearly  represents  the  actual  needs  of  the  body. 
In  other  words,  the  usual  diet,  with  its  three  hearty  meals  a  day, 
has  a  fuel  value  one  and  one  half  times  as  great  as  the  minimum 
requirement  of  the  body.  Whether  the  excess  is  or  is  not 
harmful  to  the  body  will  be  discussed  later  (see  p.  239). 

The  chief  factor  which  influences  the  amount  of  this 
minimum  fuel  value  is  the  amount  of  muscular  exercise 
taken.  Men  at  hard  labor  require  from  20  to  25  calories  per 
pound  of  body  weight,  or  even  more;  on  the  other  hand, 
during  the  marked  muscular  relaxation  of  sleep  the  require- 
ment is  reduced  to  from  10  to  11  calories.  Exposure  to  cold, 
when  not  counteracted  by  warm  clothing,  similarly  increases 
the  fuel  requirement. 

If,  then,  we  generally  eat  more  food  than  the  fuel  needs 
of  the  body  require,  what  becomes  of  the  excess  ?  This 
question  can  be  answered  only  incompletely  in  the  present 
state  of  our  knowledge.  A  portion  of  the  food  eaten  leaves 
the  body  undigested  in  the  feces ;  and  the  more  abundant 
the  diet,  the  greater  is  the  amount  lost  in  this  way.  Part 
of  the  food  also  is  destroyed  in  the  alimentary  canal,  es- 
pecially in  the  large  intestine,  by  microbic  action,  and  this 
similarly  increases  with  the  diet.  This  microbic  food  destruc- 
tion involves  the  liberation  of  heat  within  the  body  but  does 
not  yield  power  for  work,  the  excess  of  heat  being  dissipated 


222  THE  HUMAN  MECHANISM 

from  the  skin.  Again,  the  absorption  of  some  foods,  notably 
proteins  or  their  cleavage  products,  the  ammo-acids,  leads  to 
their  increased  destruction  in  the  cells  of  the  body,  just  as 
an  open  fire  burns  more  vigorously  when  new  fuel  is  added. 
Finally,  food  may  be  stored  within  the  body.  That  this  is 
true  is  shown  by  the  histories  of  prolonged  fasts  in  which 
men  and  women  have  abstained  entirely  from  food  for  more 
than  a  month.  Such  fasters  steadily  lose  weight,  showing 
that  the  body  is  consuming  its  own  substance.  We  may 
therefore  pass  to  the  consideration  of  the  storage  of  material 
capable  of  meeting  future  nutritional  needs. 

B.  THE  FOOD  RESERVE  OF  THE  BODY.   FAT. 
GLYCOGEN.    CELL  PROTEINS 

8.  The  hoarding  of  inactive  food  material.  I.  The  storage 
of  fat.  The  most  obvious  reserve  food  stored  in  the  animal 
body  is  fat,  which  may  appear  as  drops  of  oil  in  the  cyto- 
plasm of  any  cell.  Muscle  fibers,  for  example,  contain  at 
times  large  quantities  of  this  substance,  and  are  then  said 
to  have  undergone  fatty  degeneration.  Under  normal  condi- 
tions, however,  the  presence  of  considerable  quantities  of  fat 
in  muscle  fibers  or  nerve  cells  or  most  gland  cells  is  unusual. 
In  the  cells  of  connective  tissue,  on  the  other  hand,  fat  is 
readily  stored  under  normal  conditions,  and  the  adipose  tis- 
sue or  fat  of  the  body  is  simply  connective  tissue  whose  cells 
are  loaded  with  droplets  of  fat.  Figs.  90-92,  with  their  ex- 
planation, will  show  how  this  takes  place.  But  while  fat 
may  be  normally  stored  in  any  of  the  more  open  connective 
tissues,  it  is  especially  in  the  subcutaneous  tissue,  the  great 
omentum,  the  mesentery,  and  some  other  situations  that  its 
chief  storage  takes  place.  From  these  storehouses  it  is  drawn 
upon  as  a  reserve  food  material  when  the  immediate  supply 
of  food  from  the  alimentary  canal  becomes  inadequate  for 
the  work  of  the  body.  The  exact  mechanism  by  which  it  is 


NUTRITION 


223 


stored  in  a  cell  at  one  time  and 
discharged  at  another  is  not 
fully  understood.  Some  of  the 
conditions  under  which  it  is 
accumulated,  and  some  of  those 
under  which  it  disappears,  are 
known ;  but  we  do  not  know 
the  whole  story.  Some 
people  lay  up  fat  in 
larger  quantities  than 
others  on  the  same  diet 
and  apparently  while  do- 
ing the  same  amount  of 
work,  and  some  keep 
lean  under  conditions  ap- 
parently the  most  favor- 
able for  growing  fat. 

It  was  formerly  be- 
lieved, and  is  still  some- 
times supposed,  that  the 
animal  body  forms  fat 
only  from  the  fat  of  the 
food;  that  to  get  fat  we 
must  eat  fat.  This  was 
disproved  by  a  number 
of  experiments,  especially 
one  by  Liebig,  who  kept 
account  over  a  long  period 
of  the  fat  in  the  food 
supplied  to  a  cow,  and 
found  that  the  fat  given 
off  in  the  cow's  milk 
far  exceeded  that  in  her 
food.  In  another  experi- 
ment four  pigs  out  of  a 


FIG.  90 


FIG.  91 


FIG.  92 


FIGS.  90-92.  Three  successive  stages  in 

the  transformation  of  ordinary  connective 

tissue  into  adipose  tissue 

A  portion  of  the  capillary  network  is  shown, 
surrounded  hy  the  fibers,  among  which  are 
several  cells.  The  accumulation  of  fat  droplets 
in  the  cell  cytoplasm  is  shown  in  Fig.  91,  and 
the  fusion  of  these  upon  their  increase  in  size 
and  number  to  form  one  large  droplet,  sur- 
rounded hy  the  cytoplasm,  is  seen  in  Fig.  92 


224  THE  HUMAN  MECHANISM 

litter  of  eight  were  killed  and  the  total  amount  of  fat  in 
their  bodies  determined.  The  other  four  pigs  were  fattened 
for  a  time,  then  killed,  and  the  fat  in  their  bodies  eventually 
determined.  Assuming  that  the  second  set  of  four  pigs 
originally  had  the  same  quantity  of  fat  as  the  first  four,  the 
difference  between  the  two  quantities  of  fat  found  would 
give  the  quantity  of  fat  the  last  four  had  stored  up.  Mean- 
time, strict  account  had  been  kept  of  the  fat  supplied  in  the 
food  of  the  last  four,  and  it  was  shown  that  for  every  100 
parts  of  fat  fed  to  them  these  pigs  had  laid  up  472  parts  of 
fat.  They  had  evidently  manufactured  fat  from  some  substance 
other  than  the  fat  in  their  food. 

9.  Fats  can  be  stored  from  fats  and  carbohydrates  in  food. 
There  is  no  doubt  that  fat  may  be  both  stored  away  and 
manufactured  from  the  fats  in  the  food.    There  is  also  no 
doubt  that  large   quantities   of  fat  may  be   and   often  are 
manufactured  and   stored  from  the    carbohydrates   (sugars, 
starches,   etc.)   of  the  food;    so  that,   while   there  is   some 
truth  in  the  idea  that  one  may  get  fat  by  eating  fat,  it  is 
equally  true  that  we  can  get  fat  by  eating  other  foods. 

10.  Are  proteins  a  source  of  fat?    Whether  fats  are  nor- 
mally made  in  the  body  from  proteins  is  a  more  difficult 
question.    There  is  no  undisputed  case   on  record  of  such 
manufacture  and  storage ;  and  while  the  facts  do  not  yet 
justify  us   in   denying  the  possibility,   it   is  very   doubtful 
whether  such  transformation  takes  place   to  any  great  ex- 
tent, and  it  is  possible  that  in  the  mammalian  body  it  does 
not  normally  occur  at  all. 

Fats,  then,  are  manufactured  readily  from  fats  and  car- 
bohydrates and  sparingly,  if  at  all,  from  proteins.  Their 
disappearance  during  starvation,  when  they  are  drawn  upon 
to  supply  power  and  heat  for  the  body,  shows  that  they 
serve  as  a  true  reserve  food  material.  They  are  a  kind 
of  food  capital  or  hoard,  saved  and  laid  up  by  the  body 
against  a  rainy  day. 


NUTRITION  225 

11.  The  hoarding  of  inactive  food  material.  II.  The  storage 
of  glycogen.  In  many  cells  of  the  body,  but  especially  in 
those  of  the  liver  and  to  a  less  extent  in  those  of  the  skeletal 
muscles,  there  is  found  a  carbohydrate  substance  known  as 
glycogen.  This  substance  belongs  to  the  same  group  of  car- 
bohydrates as  starch  and  dextrines  (see  Chap.  VIII),  and  is 
sometimes  called  animal  starch.  Like  them  it  is  changed  into 
sugar  by  the  action  of  saliva  and  pancreatic  juice,  whence  its 
name  (7X1^5,  "sweet";  -yevrjs,  "former").  The  same  change 
occurs  on  the  death  of  the  cells  in  which  it  is  contained, 
the  sugar  thus  formed  giving  to  such  tissues  a  sweetish 
taste.  This  is  often  noticed,  for  example,  in  liver  and  in 
scallops  (the  shell  muscle  of  Pecteri).  The  total  amount 
of  glyeogen  in  the  human  body  may  exceed  700  grams 
(13  ounces),  one  half  of  which  is  concentrated  in  the 
liver  and  the  other  half  scattered  about  in  the  other  tissues 
of  the  body. 

Experiments  have  shown  that  glycogen  is  not  formed  from 
the  fat  in  food ;  that  it  is  formed  in  small  quantities  from  pro- 
tein; while  its  chief  source  is  the  carbohydrates  of  the  food. 

The  blood  may  be  said  to  be  always  sweet,  its  constant 
percentage  of  sugar  (1  to  2  grams  per  1000  cubic  centimeters 
or  0.05  ounce  per  quart  of  blood  plasma)  being  a  striking 
fact,  and  one  that  we  should  hardly  have  anticipated.  One 
might  suppose  that  the  sugar  in  the  blood  would  increase, 
as  does  the  amount  of  fat,  during  active  digestion  and  ab- 
sorption, and  that  after  digestion  had  ended,  it  would 
diminish.  As  a  matter  of  fact  the  amount  of  sugar  in  the 
blood  remains  practically  constant  both  during  and  after 
the  completion  of  digestion,  and  this  despite  the  fact  that 
the  tissues  are  constantly  abstracting  sugar  from  the  blood. 
Evidently  the  blood  must  be  supplied  with  sugar  from  some 
other  source  than  the  alimentary  canal,  and  there  must  be 
somewhere  in  the  body  a  compensating  mechanism  controlling 
the  sugar  supply  of  the  blood. 


226  THE  HUMAN  MECHANISM 

Experiments  have  shown  that  sugar  is  absorbed  from  the 
alimentary  canal  entirely  by  the  intestinal  blood  vessels.  It 
must  pass,  therefore,  through  the  liver  by  the  portal  vein 
(see  Fig.  62)  before  going  to  the  rest  of  the  body.  The 
liver,  thus  standing  at  this  great  gateway  to  the  circulation, 
would  seem  to  act  as  the  carbohydrate  storehouse,  or  savings 
bank,  of  the  body.  Any  excess  of  sugar  in  the  portal  blood 
is  there  transformed  into  glycogen  and  deposited,  saved  until 
it  is  needed,  and  then  "  paid  out,"  as  sugar,  when  the  ready 
supply  is  exhausted.  Other  tissues  doubtless  aid  in  prevent- 
ing an  undue  richness  of  sugar  in  the  blood,  acting  likewise 
as  temporary  storehouses  for  this  form  of  food. 

12.  Protein  a  source  of  sugar  to  the  body.    It  has  been 
stated  that  glycogen  may  be  formed  from  protein.    This  is 
because    the    body    can    and    does    constantly    form    sugar 
(dextrose)    from    protein.     Experiments    have    shown    that 
about  half  of  the  protein  may  in  this  way  be  transformed 
into  sugar,  the  greater  part  of  which  is  ordinarily  oxidized 
as  fuel;  but  in  case  there  is  an  excess  over  and  above  fuel 
needs,  this  excess  of  sugar  is  stored  as  glyccgen  by  the  liver 
and  other  organs,  just  as  the  excess  of  sugar  absorbed  from 
the  alimentary  canal  is  so  stored.    The  formation  of  sugar, 
and  consequently  of  glycogen  from  fat,  on  the  other  hand, 
is  negligible. 

In  this  formation  of  sugar  from  protein  the  body  obviously 
has  an  additional  means  of  supplying  its  carbohydrate  needs 
when  the  sugar  delivered  to  the  blood  by  absorption  from 
the  alimentary  canal  is  inadequate. 

13.  The   protein   reserve.    Provision   is   thus  made  for  a 
continuous  supply  of  fat  and  carbohydrate  (sugar)  between 
periods  of  absorption  of  these  foods  and  even  during  starva- 
tion.   How  are  the  protein  needs  of  the  body  met  under 
similar  conditions?    There  is  no  visible  supply  of  inactive 
protein  in  the  body  similar  to  fat  or  glycogen.    It  is  true 
that  analysis  of  the  lifeless  cell  shows  that  proteins  make 


NUTBITION  227 

up  by  far  its  largest  constituent, 1  but  there  is  no  ground 
for  thinking  that  this  cell  protein  exists  in  any  other  form 
than  as  an  active  constituent  of  the  cell  substance.  There 
is  no  evidence  of  protein  stored  simply  as  reserve  to  meet 
future  possible  protein  needs. 

And  yet,  during  starvation,  protein  is  steadily  lost  from 
the  body,  as  is  shown  by  the  fact  that  urea  and  other  protein 
derivatives  continue  to  be  eliminated  by  the  kidneys.  Nor 
can  this  loss  of  protein  be  checked  by  feeding  carbohydrates 
and  fats;  these  may  be  provided  in  the  food  in  amounts 
abundantly  sufficient  to  meet  the  fuel  demands  of  the  body, 
but  without  checking  the  loss  of  protein.  We  can  only  con- 
clude that  the  disintegration  of  protein  within  the  body  is 
an  inevitable  part  of  the  chemical  activity  of  the  cells,  and 
that  in  the  absence  of  a  supply  of  the  protein  products  of 
digestion  the  body  takes  protein  from  its  own  living  sub- 
stance. Hence  protein  becomes  an  indispensable  article  of 
diet.  The  student  will,  moreover,  recall  the  fact  that  while 
carbohydrates  and  possibly  fats  may  be  made  from  pro- 
tein, protein  cannot  be  manufactured  from  the  non-protein 
nutrients.  This  obviously  follows  from  the  fact  that  fats 
and  carbohydrates  are  lacking  in  nitrogen  and  sulphur,  two 
essential  elements  of  the  protein  molecule. 

14.  Increase  of  protein  in  the  food  increases  protein  destruc- 
tion by  the  body.  One  peculiarity  of  the  behavior  of  protein 
in  the  body  of  itself  limits  the  accumulation  of  any  large 
amount  of  storage  protein.  As  soon  as  we  increase  the  protein 
of  the  food,  there  is  an  increase  of  protein  disintegration  in 
the  body,  and  in  a  few  days  protein  disintegration  equals 

1  The  following  analysis  of  muscle  cells  (lean  of  meat)  is  typical : 

Water 75  parts 

Solids 25  parts 

Protein 21  parts 

Salts 1  part 

Fat,  connective  tissue,  etc .     .     .      2  parts 

Other  extractives  ,,,,,,, 1  part 


228  THE  HUMAN  MECHANISM 

the  greater  protein  consumption.  Instead  of  storing  the 
additional  food  protein  or  even  part  of  it  over  any  great 
length  of  time,  the  body  soon  comes  to  destroy  all  the  pro- 
tein eaten.  It  is  for  this  reason  that  while  animals  may  be 
fattened  to  a  remarkable  extent  by  proper  feeding,  it  is  not 
possible  to  secure  a  corresponding  increase  of  protein  material 
of  the  muscle,  or  lean  meat.  The  accumulation  of  protein  is 
self-limited. 

In  two  physiological  states  the  increase  of  protein  is  much 
more  marked;  namely,  during  growth  and  during  convales- 
cence from  wasting  disease  (or  after  a  period  of  prolonged 
fasting).  It  would  seem  that  there  is  a  certain  maximum 
content  of  protein-like  substances  in  the  cell,  and  that  it  is 
not  possible  by  the  most  abundant  feeding  to  increase 
this  amount. 

It  follows  from  the  above  that  very  abundant  protein 
feeding  must  result  in  the  production  of  increased  protein 
waste  within  the  body.  In  the  first  place,  the  greater  the 
quantity  of  protein  fed,  the  greater  will  be  the  microbic 
destruction  of  protein  within  the  intestine  and  especially  in 
the  large  intestine.  Not  only  is  the  protein  so  destroyed 
largely  useless  to  the  body,  but,  in  so  far  as  its  microbic 
destruction  involves  putrefactive  changes,  harmful  products 
may  be  formed  from  it.  In  the  second  place,  that  portion  of 
the  protein  which  escapes  microbic  action  and  is  absorbed 
into  the  blood  in  the  form  of  digestive  products  (amino-acids 
and  peptids)  disintegrates  in  the  cells  with  the  formation  of 
wastes.  Both  these  processes  increase  the  amount  of  waste 
to  be  eliminated,  chiefly  by  the  kidneys.  It  has  been  urged 
that  this  overburdens  the  kidneys  and  causes  disease  of 
these  organs.  While  convincing  proof  has  perhaps  not  been 
given  that  a  healthy  kidney  may  be  injured  in  this  way, 
it  is  certain  that  a  diseased  or  even  a  temporarily  impaired 
kidney  may  suffer  when  such  excessive  work  is  thrown 
upon  it. 


NUTRITION  229 

C.  FOOD  AS  THE  MATERIAL  FOR  GROWTH,  REPAIR,  AND 

THE  MANUFACTURE  OF  SPECIAL  PRODUCTS  OF 

CELL  ACTIVITY 

15.  Complexity  of  the  chemical  composition  of  living  cells 
and  of  the  products  of  their  manufacture.  In  the  first  sub- 
division of  this  chapter  we  have  considered  food  as  fuel. 
We  are  now  in  a  position  to  consider  some  of  the  more 
important  features  of  the  other  great  function  of  food, 
namely  as  the  material  for  the  growth  and  repair  of  living 
cells  and  for  the  manufacture  of  the  special  products  of 
cell  life  —  the  secretions  (internal  and  external),  the  hor- 
mones, and  all  other  substances  produced  by  the  body  for 
special  purposes. 

The  living  cell  is  an  extremely  complex  machine  into  the 
construction  of  which  enter  numerous  compounds  of  diverse 
chemical  nature.  Formerly  there  was  a  tendency  to  regard 
the  cell  as  composed  essentially  of  protein;  but  the  increase 
of  our  knowledge  has  shown  that  there  are  other  essential 
constituents,  notably  (in  addition  to  water  and  inorganic 
salts)  a  group  of  compounds  known  as  lipoids,  or  lipins, 
substances  which  more  or  less  resemble  fats  in  their  physi- 
cal characters,  although  not  always  in  chemical  structure. 
The  cell  nucleus  also  contains  special  material  of  still  dif- 
ferent chemical  composition.  The  chemical  properties '  and 
the  physiological  significance  of  these  cell  components  are 
far  too  complicated  subjects  for  discussion  in  this  work ;  we 
merely  wish  to  emphasize  the  complexity  of  the  mixture  and 
the  variety  of  chemical  compounds  concerned.  (See  p.  42.) 

We  are  impressed  with  the  same  diversity  of  chemical 
structure  in  the  secretions,  hormones,  and  other  material 
manufactured  by  the  body  for  special  purposes.  The  stu- 
dent has  only  to  recall  the  examples  of  these  already  men- 
tioned —  the  enzymes  of  the  digestive  juices ;  the  internal 
secretions  of  the  adrenals,  thyroids,  pituitary,  and  pancreas ; 


230  THE  HUMAN  MECHANISM 

secretin  and  other  hormones;  mucin;  hemoglobin — to  realize 
that  the  food  must  furnish  material  out  of  which  to  manu- 
facture compounds  of  the  greatest  variety  of  chemical  struc- 
ture ;  and  for  this  purpose  the  greatest  variety  of  material 
must  be  furnished  in  the  food. 

16.  The  unique  position  of  protein.    Considerations  such  as 
the  above  give  a  glimpse  into  the  unique  value  of  protein 
food.     While  all  forms  of  carbohydrate  yield  the  body,  for 
the  greater  part,  only  dextrose,  and  the  fats  yield  only  fatty 
acids  and  soaps,  all  of  them  closely  similar  in  structure,  pro- 
tein yields  amino-acids  of  the  greatest  diversity  of  chemical 
structure.    The  possibilities  of  chemical  construction,  or  syn- 
thesis (as  it  is  generally  called),  are  thereby  greatly  increased. 
Only  a  chemically  complex  food  like  protein  could  serve  for 
the  construction  of  the  proteins  of  the  living  cell  and  for  the 
formation  of  the  varied  products  of  cell  manufacture.   Review 
in  this  connection  section  15  of  Chapter  VIII. 

Protein  is  also  unique  among  the  nutrients  in  the  fact 
that  the  body  can  make  other  nutrients  from  it.  It  is  a  well- 
established  fact  that  large  quantities  of  sugar  (dextrose)  may 
be  made  from  protein,  and  we  can  therefore  understand  how 
a  dog  living  on  fat  and  the  leanest  sort  of  meat  (protein) 
can  do  without  carbohydrate  in  the  diet.  It  is  also  possible 
that  at  least  small  amounts  of  fat  may  be  derived  from  pro- 
tein through  this  intermediate  stage  of  sugar,  for  fat  may  be 
made  from  sugar. 

17.  Variations  in  the  nutritional  value  of  individual  pro- 
teins.    Until   comparatively  recent  times   all  food  proteins 
were  regarded  as  having  equal  value  in  nutrition,  with  the 
single  exception  of  gelatin,  which  has  long  been  known  to 
be  incapable  of  meeting  the  protein  needs  of  the  body.    The 
discovery  that  some  food  proteins  are  lacking  in  one  or  more 
of  the  amino-acids,  and  that  the  same  amino-acid  may  occur 
in  very  small  amounts  in  one  protein  and  very  large  amounts 
in  another,  suggested  to  two  American  physiologists,  Mendel 


NUTRITION  231 

and  Osborne,  the  idea  that  different  proteins  may  have  very 
different  values  in  nutrition.  They  therefore  fed  rats  and 
mice  on  diets  of  abundant  fuel  value  and  containing  all  the 
non-protein  constituents  of  the  diet,  but  containing  only  one 
protein.  It  was  found  that  some  proteins  failed  entirely  to 
nourish  the  animal,  as  shown  by  the  steady  loss  of  weight ; 
others  would  keep  an  adult  animal  in  good  condition  with 
no  loss  of  weight,  but  did  not  provide  the  material  for  the 
normal  growth  of  young  animals ;  other  proteins  not  only 
maintained  the  normal  weight  of  the  adult  but  a  young 
animal  fed  on  them  would  grow  in  a  perfectly  normal  man- 
ner. We  must  therefore  distinguish  between  (1)  proteins 
which  may  provide  for  both  growth  and  maintenance,  (2)  pro- 
teins which  will  provide  for  maintenance  but  not  for  growth, 
and  (3)  proteins  which  will  provide  for  neither  maintenance 
nor  growth. 

Further  study  showed  that  the  nutritional  limitations  of 
the  last  two  classes  of  proteins  are  due  to  the  fact  that  they 
are  lacking  in  certain  amino-acids  or  else  contain  them  in 
very  small  amounts;  for  if  these  amino-acids  were  added  to 
the  diet,  growth  and  maintenance  became  normal.  The  rea- 
son for  this  becomes  clear  on  the  assumption  already  made 
in  our  discussions  of  digestion  and  nutrition,  that  the  value 
of  protein  as  food  lies  in  the  fact  that  it  yields  a  great 
variety  of  amino-acids,  each  necessary  to  some  chemical 
manufacturing  process  of  the  living  cell. 

18.  The  value  of  the  mixed  diet.  As  a  matter  of  fact  no 
one  tries  to  live  on  a  single  protein.  Meat  contains  at  least 
two ;  eggs,  three  or  more ;  milk,  two ;  the  cereals,  two  or 
more  each.  By  taking  a  mixture  of  these  in  our  food,  the 
deficiency  of  one  protein  in  amino-acids  is  made  up  by  the 
excess  of  the  same  acid  in  another.  For  this  reason  we  can 
completely  meet  the  protein  needs  of  the  body  on  a  mixed 
diet  with  a  far  smaller  total  intake  of  protein  than  if  the 
diet  contained  only  one  protein. 


232  THE  HUMAN  MECHANISM 

The  same  consideration  applies  in  a  larger  way  to  the  food 
as  a  whole.  Some  foods,  like  meat,  are  chiefly  protein; 
others,  like  the  cereals,  have  an  excess  of  starch,  while 
others,  like  butter  or  olive  oil,  are  chiefly  fat.  A  diet  com- 
posed of  several  kinds  of  food,  that  is,  a  mixed  diet,  is  more 
likely  to  avoid  an  excess  of  any  one  nutrient  than  when 
any  one  food  unduly  preponderates. 

19.  Other  indispensable  constituents  of  the  food.    I.  Inor- 
ganic salts.    In   addition   to   the  proteins,  fats,   and   carbo- 
hydrates, which  together  make  up  almost  the  whole  (96  to  98 
per  cent  or  even  more)  of  the  food  we  eat,  two  other  groups 
of  substances  are  required  in  much  smaller  quantities,  but 
they  are  none  the  less  absolutely  indispensable.    The  first 
of  these  is  the  group  of  inorganic  salts.    In  the  body  are 
found  chlorides,  carbonates,  and  phosphates  of  sodium,  potas- 
sium,  calcium,  and    magnesium.    These   occur  both  in  the 
living  cells  and  in  the  blood  and  lymph,  and  they  are  con- 
stantly being  removed  from  the  blood  in  the  urine  and  per- 
spiration.   This  loss  must  be  made  good  by  the  food.    Most 
foods  contain  salts,  and  our  usual  food  contains  most  of  the 
inorganic  salt  necessary  for  making  good  the  loss.    The  table 
salt  used  in  cooking  and  to  develop  the  flavor  of  food  at 
tablets  for  the  greater  part  in  excess  of  the  actual  needs  of 
the  body,  the  excess  being  promptly  excreted  by  the  kidneys. 
The  addition  of  some  salt,  however,  seems  to  be  necessary. 
The  craving  of  herbivorous  animals  for  salt  in  which  their 
food  is  deficient  is  well  known,  and  in  parts  of  India  salt 
famines  have   occurred  during  which  the  price  of  salt  was 
higher  than  that  of  gold. 

20.  Other  indispensable  constituents  of  the  food.  II.  "  Vita- 
mines.  "    Finally,  it  is  known  that  certain  other  compounds, 
found    in    small    quantities    in    many   foods,    are    necessary 
for  adequate    nourishment.     The   exact  chemical   nature  of 
these  substances  is  still  a  matter  of  investigation,  but  it  is 
known  that  they  are  neither  protein,  fat,  carbohydrate,  nor 


NUTRITION  233 

inorganic  salts.  They  occur  in  the  outer  layers  of  cereal 
grains,  such  as  wheat,  rice,  oats,  etc. ;  they  are  also  present 
in  most  fresh  vegetables  and,  in  smaller  quantities,  in  meats, 
eggs,  and  milk;  and  they  are  produced  by  the  yeast  plant 
during  its  active  growth.  Hence  they  may  be  extracted  from 
yeast  cakes.  To  them  the  general  name  of  vitamines  has 
been  given. 

In  many  Eastern  countries,  where  rice  forms  the  chief 
article  of  diet,  a  severe  disease  known  as  beriberi  is  more 
or  less  common.  It  is  characterized  by  grave  disorders  of 
nutrition,  and  in  severe  forms  the  nerve  fibers  undergo 
degeneration,  so  that  paralysis  of  the  skeletal  muscles 
develops.  It  was  found  that  beriberi  occurred  chiefly  among 
those  who  used  polished  rice,  that  is,  rice  from  which  the 
dark  outer  portion  of  the  grain  had  been  removed  in  the 
process  of  milling,  in  order  to  give  a  whiter  rice  grain,; 
it  seldom  developed  in  those  using  the  whole  rice  grain 
(that  is,  the  unpolished  rice).  It  was  furthermore  found 
that  from  the  rice  polishings  something  could  be  extracted 
which  when  administered  in  very  small  quantities  would 
cure  the  disease.  Finally,  it  was  found  that  a  similar  disease 
(polyneuritis)  could  easily  be  induced  in  fowls  by  feeding 
them  on  a  diet  consisting  solely  of  polished  rice,  but  that 
it  did  not  develop  when  the  extract  of  rice  polishings  was 
administered  to  the  fowls  even  though  their  food  other- 
wise consisted  wholly  of  polished  rice.  This  extract  would, 
moreover,  cure  the  disease  when  it  had  once  developed. 

Whether  one  group  or  more  than  one  group  of  compounds 
is  concerned  here  is  not  known.  It  is  clear,  however,  that 
we  have  in  the  above  facts  proof  of  some  essential  constitu- 
ent or  constituents  of  the  diet  other  than  the  usual  nutrients. 
These  vitamines  seem  to  occur  abundantly  in  most  fresh 
fruits  and  freshly  cooked  vegetables  and  in  the  outer  por- 
tion of  most  cereal  grains.  They  are  destroyed  by  very  high 
temperatures,  especially  those  used  in  sterilizing  canned 


234  THE  HUMAN  MECHANISM 

foods,  and  they  are  largely  removed  from  the  cereal  grains 
in  the  attempt  of  the  miller  to  produce  the  whitest  possible 
flour  or  rice,  for  this  means  the  removal  of  the  outer  por- 
tion (bran)  of  the  grain  with  its  vitamines.  It  follows  that 
"  whole  wheat "  flour  or  graham  flour  contains  these  sub- 
stances, while  very  white  flour  is  deficient  in  them;  and 
we  accordingly  find  that  the  same  disease  (beriberi)  has 
occurred  in  Newfoundland,  where  a  community  was  shut 
off  during  whiter  from  its  usual  food  supply  and  white  bread 
constituted  for  too  long  a  time  the  chief  food.  A  similar 
and  probably  identical  disease  has  been  found  among  people 
living  exclusively  upon  canned  goods,  the  sterilization  by 
high  temperatures  having  destroyed  the  vitamines. 

In  the  days  of  sailing  vessels,  scurvy,  a  disease  of  mal- 
nutrition, often  developed  on  long  voyages,  despite  a  diet 
which  contained  an  abundance  of  protein,  fat,  carbohydrate, 
and  salt;  and  it  was  found  that  this  disease  could  be  pre- 
vented by  the  use  of  fresh  limes  or  freshly  cooked  vege- 
tables. There  is  little  doubt  that  here  again  we  are  dealing 
with  a  disease  analogous  to  beriberi. 

In  all  the  above  cases  it  must  be  clearly  understood  that 
there  is  no  harmful  constituent  in  the  foods  mentioned  — 
canned  foods,  polished  rice,  white  bread,  and  the  like.  The 
trouble  lies  in  the  absence  from  the  food  of  an  essential  con- 
stituent of  the  diet.  No  harm  would  result,  for  example, 
from  a  diet  of  canned  meat,  white  bread,  and  fresh  vege- 
tables ;  for  the  fresh  vegetables  would  supply  the  necessary 
vitamines.  It  is  only  when  the  diet  consists  almost  exclu- 
sively of  foods  deficient  in  vitamines  that  trouble  results. 

The  physiological  action  of  these  vitamines  is  not  yet  clear, 
but  we  are  probably  not  far  from  the  truth  if  we  regard  them 
as  furnishing  the  body  with  some  material  indispensable  for 
carrying  out  the  processes  of  chemical  manufacture.  Though 
required  in  much  smaller  quantities,  they  are  as  necessary  to 
these  processes  as  the  amino-acids  themselves. 


NUTEITION  235 

D.  THE  PROPER  DAILY  INTAKE  OF  PROTEIN 

21.  The    economic    and    the    physiological    question.     The 
proper  amount  of  protein  in  the  diet  is  both  economically 
and  physiologically  important.    Since  foods  rich  in  protein  — 
meats,  eggs,  dairy  products,  etc.  —  are  among  the  more  ex- 
pensive foods,  it  is  often  for  a  family  with  limited  income 
a  practical  question  how  much  of  these  foods  must  be  used 
to  assure  proper  nourishment  of  the  body.    In  this  work  we 
are  more  directly  concerned  with  the  physiological  effects  of 
low,  moderate,  and  abundant  protein  diet,   but  the  answer 
to  this  question  also  gives  the  answer  to  the  economic  ques- 
tion, since  the  problem  in  the  latter  case  is  to  keep  down 
the  consumption  of  the  more  expensive  foods  to  the  level 
which  is  consistent  with  adequate  nutrition. 

It  is  comparatively  easy  to  determine  whether  the  fuel 
value  of  the  diet  is  adequate.  If  it  is  insufficient,  loss  of 
weight  inevitably  results ;  if  it  is  excessive,  and  especially 
if  it  is  excessive  in  fat  and  carbohydrate,  there  is  apt  to  be 
increase  of  weight.  An  equilibrium  of  total  intake  and  out- 
put for  months  usually  indicates  that  the  fuel  needs  of  the 
body  are  being  met.  Equality  of  intake  and  output  of  pro- 
tein, on  the  other  hand,  does  not  prove  that  the  protein  of 
the  diet  is  what  it  should  be,  for  the  body  breaks  down  all 
the  protein  it  consumes  whether  the  amount  be  excessive  or 
not.  We  can,  however,  determine  by  dietary  studies  what 
is  the  usual  consumption  of  protein  by  different  classes  of 
people  and  also  what  is  the  lowest  intake  upon  which 
protein  equilibrium  may  be  maintained  in  the  body. 

22.  The  usual  and  the  minimum  intake  of  protein.    When 
the   choice   of  food  is  not  restricted  by  economic  or  other 
consideration,  but  is  determined  solely  by  appetite   or  the 
feeding  customs  of  one's  home   or  community,  the  protein 
intake  of  an  adult  healthy  man  usually  varies  between  100 
and  150  grams    daily.    This   is    equivalent  to  from   500  to 


236  THE  HUMAN  MECHANISM 

750  grams  (1  to  li  pounds)  of  lean  meat,  although  of  course 
the  protein  is  not  all  taken  in  the  form  of  meat.  On  the 
other  hand,  experiments  have  shown  that  men  may  live  for 
years  in  good  health  on  a  protein  intake  of  from  40  to 
50  grams  daily  without  loss  of  protein  from  the  body. 

If  then  an  adult  man  can  maintain  puotein  equilibrium  on 
from  40  to  50  grams  of  protein  daily,  but  ordinarily  con- 
sumes two  to  three  times  this  quantity,  the  question  arises 
whether  the  additional  50  to  100  grams  are  in  any  way 
harmful.  Many  students  of  this  subject  have  strongly  taken 
the  position  that  such  is  the  case,  and  there  can  be  no 
question  that  the  health  of  many  people,  especially  when 
leading  sedentary  lives,  has  been  greatly  improved  by  re- 
ducing the  consumption  of  protein  to  60  or  70  grams,  or 
even  to  40  or  50  grams  daily.  To  what  is  this  improvement 
due  ?  Is  it  because  the  handling  of  so  much  protein  by  the 
adult  is  necessarily  harmful  ?  (See  p.  239.) 

23.  Possible  harm  and  possible  advantage  in  an  abundant 
protein  diet.  We  can  readily  see  at  least  two  ways  in.  which 
the  greater  protein  intake  may  be  harmful.  In  the  first  place, 
it  involves  greater  danger  of  incomplete  protein  digestion  in 
the  small  intestine  and  the  consequent  delivery  by  peristalsis 
of  excessive  amounts  of  protein  to  undergo  microbic  putre- 
faction in  the  large  intestine.  In  general  the  presence  of  a 
decidedly  offensive  odor  to  the  feces  suggests  that  more  pro- 
tein1 is  being  eaten  than  can  be  properly  digested,  and  justi- 
fies at  least  an  experimental  reduction  in  the  protein  of  the 
food.  It  must,  however,  be  remembered  that  putrefactive 
odors  may  be  due  to  other  causes  than  excessive  protein 
diet  (impaired  digestion  of  fats,  for  example)  and,  on  the 
other  hand,  there  may  be  excessive  putrefaction  and  yet  the 
feces  have  no  very  offensive  odor  because  the  compounds 
responsible  therefor  have  been  destroyed  within  the  body. 

1  The  substances  responsible  for  the  offensive  odor  are  almost  entirely 
derivatives  of  protein. 


NUTRITION  237 

In  the  second  place,  the  larger  protein  diet  with  its  in- 
crease of  protein  wastes  in  the  body  itself  (as  contrasted 
with  the  alimentary  canal)  involves  a  greater  burden  on  the 
organs  of  excretion.  This  burden  may  fall  not  alone  on  the 
kidneys,  which  finally  discharge  these  wastes  from  the  body, 
but  also  upon  other  organs  in  which  the  waste  products  are 
prepared  for  final  removal  from  the  blood  by  the  kidneys. 
Convincing  proof  has,  however,  not  been  given  that  these 
organs,  when  in  a  healthy  condition,  are  injured  by  the  work 
of  caring  for  more  than  the  waste  of  a  low  protein  diet. 
A  somewhat  analogous  case  is  that  of  muscular  activity. 
This,  too,  must  be  limited  or  even  given  up  altogether  in 
some  diseased  conditions  lest  some  undue  burden  be  placed 
upon  the  organism ;  but  in  health  the  body  is  actually  bene- 
fited by  the  "  burden  "  of  even  vigorous  muscular  activity. 

The  further  question  then  arises  whether  there  is  any 
possible  advantage  in  a  liberal  protein  diet.  It  is  certainly 
not  needed  for  power  or  for  fuel;  it  may,  however,  be  plau- 
sibly urged  that  thereby  we  insure  an  abundance  of  each 
amino-acid  needed  for  the  formation  of  the  innumerable 
products  of  chemical  manufacture  in  the  body.  When  an 
engineer  builds  a  bridge,  he  does  not  build  it  just  strong 
enough  to  sustain  the  expected  load ;  he  allows  a  liberal 
"  margin  of  safety."  Similarly,  it  is  not  a  desirable  economic 
condition  when  one's  income  each  week  is  just  enough  to 
meet  necessary  expenses,  for  this  does  not  allow  for  the 
unexpected  emergency  which  we  cannot  foresee.  So  it  has 
been  urged  and,  it  would  seem,  reasonably  urged  that  it  is 
better  not  to  diminish  protein  intake,  as  a  rule,  to  the  irre- 
ducible minimum  of  40  to  50  grams  daily.  While  100  to  150 
grams  is  almost  certainly  far  more  than  is  necessary  to  secure 
the  proper  margin  of  safety,  it  may  well  be  wiser  to  use  20 
or  more  grams  above  the  minimum ;  that  is  to  say,  a  protein 
intake  of  70  grams  corresponds  with  what,  in  the  present  state 
of  our  knowledge,  may  be  regarded  as  a  conservative  estimate. 


238 


THE  HUMAN  MECHANISM 


Infants  and  rapidly  growing  children  need  relatively  more 
protein  than  adults.  The  protein  of  the  usual  adult  diet 
makes  about  13  to  15  per  cent  of  the  total  fuel  value  of  the 
food ;  in  milk,  the  sole  diet  of  a  baby,  it  makes  20  to  25  per 
cent.  A  similar  thing  is  true  of  the  diet  during  convalescence 
from  wasting  diseases ;  such  a  diet  should  be  as  rich  in  pro- 
tein as  is  consistent  with  its  proper  digestion  and  utilization 
by  the  body. 

24.  Food  values  of  some  common  foods.  The  folio  whig 
table  (from  Joslin)  will  be  found  useful  in  forming  an  esti- 
mate of  the  content  of  certain  foods  in  protein,  fat,  and 
carbohydrate,  and  also  of  the  fuel  value  of  these  foods. 


30  GRAMS  (OR  1  OUNCE)  CONTAIN 
APPROXIMATELY 

PROTEIN 

FAT 

CARBOHY- 
DRATE 

CALORIES 

Oatmeal,  dry  weight    .... 
Cream,  40  per  cent       .... 
Cream,  20  per  cent      .... 
Milk      

Grams 
5 
1 
1 
1 

Grams 
2 
12 
6 
1 

Grams 
20 
1 
1 
1  5 

120 
120 
60 
20 

Brazil  nuts               .... 

5 

20 

2 

210 

Oysters  (six) 

6 

1 

4 

50 

Meat  (uncooked,  lean)      .     . 
Meat  (cooked,  lean)     .... 
Bacon   

6 
8 
5 

3 
5 
15 

0 
0 
0 

50 
75 
155 

Ego-  (one)  . 

6 

6 

o 

75 

Vegetables  (5  and  10  per  cent 

rrpOUDS) 

0  5 

o 

1  or  2 

6  or  10 

Potato  

1 

o 

6 

25 

Bread    

3 

o 

18 

90 

Butter 

o 

25 

o 

225 

Fish       

5 

0 

o 

20 

Broth    

0  7 

0 

o 

3 

Small  orange  or  half  a  grape- 
fruit    

o 

0 

10 

40 

An  individual  "at  rest"  requires  about  25  calories  per  kilogram 
(2.2  Ib.)  body  weight  per  24  hours,  equivalent  to  approximately  1  calorie 
per  kilogram  per  hour. 


NUTRITION  239 

25.  Example  of  a  diet  of  moderately  low  protein  and  fuel 
value.  The  following  table  gives  an  example  of  three  meals 
which  would  give  the  moderate  protein  intake  referred  to  on 
page  237.  The  fuel  value  also  corresponds  approximately, 
for  a  man  of  150  to  160  pounds,  to  the  fuel  value  of  13.5 
calories  per  pound  of  body  weight  referred  to  on  page  221. 

Breakfast.    Bread,  38.7  grams ;  tea,  146  grams. 

Lunch.   Bread, 97.5  grams;  butter, 31. 5  grams ;  sweet  potato,  108.7  grams; 

spaghetti,  82.5  grams ;  peaches,  89.4  grams  ;  coffee,  210  grams  ;  sugar, 

21  grams. 
Dinner.    Bread,  75  grams ;  butter,  21.5  grams ;  roast  beef,  116  grams ; 

lemon  pie,  188.5  grams ;  coffee,  210  grams ;  sugar,  21  grams. 

Protein  in  food 70  grams 

Fuel  value 2334  calories 

30  grams  =  1  ounce,  or  yL  pint. 


CHAPTER  XIV 

SENSE  OKGANS  AND  SENSATIONS 

1.  The  human  mechanism  a  conscious  mechanism.    Thus 
far   we   have   repeatedly   compared   the   human   mechanism 
with  lifeless  mechanisms,   and  the  points  of  similarity  are 
most  interesting  and  instructive.    In  the  supply  of  power, 
the  elimination  of  wastes,  the  interdependence  and  coopera- 
tion  of   parts,   the    adjustment   to   the    changing   conditions 
of  work,  and  in  many  other  respects  the  resemblance  holds 
good.    But  in   one    respect  there   is  no   likeness   whatever. 
When  a  human  mechanism  is  not  in  good  working  order  or 
is  tired,  it  may  be  aware  of  the  fact;  when  an  engine  is 
damaged  in  any  way,  the  engine  does  not  know  it.    Events 
taking  place  in  the  living  animal  body  arouse  in  it,  and  in 
it  only,  conscious  sensations. 

Sensations  are  always  called  forth  by  the  condition  of 
some  organ  or  by  the  condition  of  the  body  as  a  whole. 
When  several  hours  have  passed  since  the  taking  of  food, 
we  feel  hungry;  or  of  drink,  we  feel  thirsty;  when  any- 
thing touches  the  skin  a  sensation  of  touch  is  aroused;  if 
it  presses  very  hard,  that  part  of  the  skin  feels  painful ;  if 
the  tongue  is  acted  upon  by  sugar  or  salt,  we  get  a  sensa- 
tion of  taste ;  if  light  enters  the  eye,  it  produces  conditions 
hi  that  organ  which  arouse  in  us  sensations  of  color.  In  all 
these  cases  the  conscious  sensation  is  due  to  the  condition  of 
some  part  of  the  body. 

2.  The  reference  of  sensations.     Sometimes  we  refer  the 
sensation  to  the  part  of  the  body  which   is   first  affected, 
or  to  the  body  as  a  whole,  and  sometimes  we  refer  it  to 

240 


SENSE  ORGANS  AND  SENSATIONS  241 

external  objects.  Thus,  if  in  driving  a  nail  the  hammer 
misses  the  nail  and  hits  a  finger,  we  refer  the  pain  to  the 
finger  and  not  to  the  hammer ;  and  we  similarly  refer  sensa- 
tions of  hunger  and  thirst  to  the  body  and  not  to  external 
objects.  If,  on  the  other  hand,  the  skin  is  cooled  by  a 
piece  of  ice,  we  do  not  say  that  the  skin  is  cold,  but  that 
the  ice  is  cold;  we  refer  the  sensation  to  the  external  object 
which  causes  it,  not  to  the  skin  in  which  it  actually  origi- 
nates. In  the  case  of  the  sense  of  sight,  this  reference  of 
the  sensation  to  the  external  object  which  sends  light  into 
the  eye  is  so  complete  that  unless  we  stop  and  reflect  upon 
it  we  do  not  realize  that  it  is  the  condition  of  the  eye  of 
which  we  are  conscious  rather  than  the  condition  of  the 
external  object  at  which  we  are  looking. 

3.  Sense  organs.    A  few  sensations,  like  pain,  are  aroused 
by  the   condition   of   most,   if  not   all,   parts   of  the   body; 
there  is  no  one  organ  set  apart  to  produce  them.    Some,  like 
hunger,   although  at  times  more  or  less  general  in   origin, 
are  commonly  aroused  by  the  condition  of  some  one  organ  1 
which  ordinarily  performs  other  functions.    Other  sensations 
arise  in  organs  set  apart  for  the  purpose  and  constructed  to 
react  to  only  one  kind  of  stimulus  (special  sense  organs,  or 
organs  of  special  sensation).    To  this  latter  class  belong  tl.e 
eye,  the  ear,  the  olfactory  mucous  membrane  of  the  nose, 
the  touch  organs  in  the  skin,  etc.    We  therefore  speak  of 
general  sensations  and  special  senses,  although  no  sharp  line 
of  division  can  be  drawn  between  the  two. 

4.  The  brain  the  seat  of  sensation.    In  all  cases,  however, 
the  sensation,   although  originating  elsewhere,  is  developed 
in  the  brain  and  not  in  the  sense  organ.    If  the  optic  nerve 
be  cut,  blindness  ensues,  although  light  falling  on  the  retina 
produces  the  same  effect  in  the  eye  itself  as  when  the  nerve 
is  intact ;  it  even  starts  nervous  impulses  toward  the  brain  ; 
but,  since  these  impulses  go  no  farther  than  the  cut,  they 

1  In  the  case  of  hunger,  the  stomach. 


242  THE  HUMAN  MECHANISM 

excite  no  sensation  of  light.  And  the  same  thing  is  true  of 
other  sensations.  Conversely,  after  the  amputation  of  a  limb 
it  often  happens  that  sensations  are  felt,  as  if  they  came 
from  the  lost  member.  In  this  case  the  stump  of  the  cut 
nerve  is  stimulated  in  some  way,  and  the  impulses  thus 
sent  to  the  brain  excite  the  same  sensations  as  if  they  came 
from  the  usual  endings  of  the  nerve.  When  one  hits  his 
"  funny "  or  "  crazy "  bone  (that  is,  directly  stimulates  the 
ulnar  nerve)  the  sensations  developed  in  the  brain  may  be 
referred  to  the  fingers  in  which  the  nerve  originates. 

In  the  development  of  every  sensation,  therefore,  we  have 
to  distinguish  between  (1)  what  takes  place  in  the  sense 
organ  or  end  organ,  (2)  the  passage  of  a  nervous  impulse 
from  this  organ  to  the  central  nervous  system,  and  (3)  the 
events  which  the  arrival  of  the  nervous  impulse  excites  in 
the  brain.  It  is  only  the  last  (3)  that,  strictly  speaking,  we 
can  call  sensation.  The  sense  organs  and  their  afferent  fibers 
are  merely  tributary  mechanisms  which  serve  to  excite  the 
sensations  in  the  brain.  We  are  not  aware  that  it  is  the 
brain  which  is  thus  active,  for  we  refer  the  sensation  either 
to  the  organ  or  to  some  external  object. 

5.  The  sense  of  sight ;  the  eye.    Sight  is  one  of  the  most 
highly  specialized  of  the  senses.    The  eye  is  the  only  organ 
in  which  originate  sensations  of  light  or  color,  and  it  is  a 
wonderfully  constructed  apparatus,  the  function  of  which  is 
to  stimulate  the  optic  nerve  by  rays  of  light.    It  is  essen- 
tially  a   living   camera   in   which,   by  means   of   a   lens,   an 
image  of  things  around  us  is  formed  upon  the  retina;  just 
as  in  the  photographer's  camera  the  lens  forms  an  image  on 
the  ground  glass  or  on  the  sensitive  plate  or  film. 

6.  Structure   of   the  eye.    The   eyeball   consists   of  three 
concentric  coats  surrounding  and  inclosing  transparent  sub- 
stances through  which  rays  of  light  pass  to  the  retina.    The 
outer,  or  sclerotic,  coat  (the  white  of  the  eye)  is  composed  of 
very  tough,  dense  connective  tissue,  and  forms  the  protecting 


SENSE  ORGANS  AND  SENSATIONS  243 

covering  of  the  eye.  Over  a  small  area  in  front  this  coat  is 
transparent,  and  this  part  of  it  is  known  as  the  cornea.  In- 
side the  sclerotic  is  the  middle  coat,  or  choroid,  richly  sup- 
plied with  blood  vessels  and  containing  in  its  connective 
tissue  large  quantities  of  black  pigment,  which  prevents  the 
passage  of  light  into  the  eyeball  except  through  the  cornea. 
The  choroid  lines  the  sclerotic  everywhere  except  in  front, 
where  in  the  region  of  the  cornea  it  leaves  the  sclerotic  and 
projects  toward  the  long  axis  of  the  eye  as  a  kind  of  cur- 
tain, the  iris  —  that  part  of  the  eye  which  is  black  or  gray 
or  blue.  The  pupil  is  the  dark  round  opening,  or  hole,  in 
the  iris.  Immediately  inside  the  choroid  is  the  third  and 
innermost  coat,  the  retina.  This  is  a  thin  membrane,  not 
more  than  one  eightieth  of  an  inch  in  thickness,  and  lining 
the  chamber  of  the  vitreous  humor  as  far  forward  as  the 
ciliary  region  (Fig.  93).  The  retina  is  the  part  of  the  eye 
sensitive  to  the  stimulation  of  light.  Here  also  begin  the 
fibers  of  the  optic  nerve,  which  passes  through  and  perforates 
the  choroid  and  sclerotic  coats  behind  on  its  way  from  the 
retina  to  the  brain.  These  and  other  parts  of  the  eye  may 
be  easily  seen  by  dissecting  the  eye  of  an  ox  or  sheep. 

7.  The  lens  and  the  muscle  of  accommodation.  Immedi- 
ately behind  the  pupil  is  the  lens,  a  biconvex,  transparent, 
compressible,  and  elastic  body  fastened  by  a  circular  liga- 
mentous  sheet  to  the  choroid  coat  immediately  above  and 
behind  the  iris.  The  lens  and  its  suspensory  ligamentous 
sheet  thus  divide  the  eye  into  two  distinct  chambers:  the 
one,  in  front  of  the  lens  and  behind  the  cornea,  filled  with 
a  watery  fluid,  the  aqueous  humor;  the  other,  behind  the 
lens  and  surrounded  by  the  retina,  filled  with  a  jellylike, 
transparent  substance,  the  vitreous  humor  (Figs.  93,  96). 

The  elastic  choroid  coat  is  not  long  enough  to  reach 
around  and  inclose  the  vitreous  humor  without  stretching, 
and  hence  it  constantly  exerts  a  steady,  elastic  pull  or  ten- 
sion on  the  ligament  of  the  lens.  This  tension  flattens  the 


244 


THE  HUMAN  MECHANISM 


compressible  lens  (that  is,  makes  it  less  convex),  and 
the  lens  is  always  in  this  flattened  condition  in  the  resting 
eye;  for  example,  when  one  is  asleep.  The  same  condition 
should  obtain,  as  we  shall  learn,  whenever  we  are  looking 
at  distant  objects. 

The   pull  of  the  tense   choroid  on  the  lens  is,  however, 
overcome    at   times   by  the   action   of   the    sheetlike   ciliary 

muscle.  The  fibers  of  this  pe- 
culiar muscle  originate  in  the 
sclerotic  coat  around  and  just 
outside  the  cornea,  and  diverge 
radially  outward  and  backward 
to  end  in  the  choroid  beyond 
the  attachment  of  the  suspen- 
sory ligament  of  the  lens. 
^Suspensory  Fig.  94  shows  how  the  con- 
traction of  this  muscle,  fixed 
as  it  is  near  the  cornea,  must 
draw  the  choroid  forward  and 
so  ease  the  pull  of  the  latter 
FIG.  93.  Vertical  section  through  on  the  ligament  of  the  lens, 
the  ciliary  region  of  the  eye  When  this  happens,  the  lens, 

Showing  the  structures  concerned  in  owing  to  its  OW11  elasticity, 
accommodation  (see  sect.  7).  This  .  .  ,  , 

should  be  compared  with  the  perspec-    assumes  its  independent  (more 

tive  view  into  the  hemisphere  of  the      convex^)   shape 

The  curvature  of  the  lens  is 


Ligament 


eyeball,  shown  in  Fig.  167 


thus  variable,  and  is  determined  by  the  action  of  this  muscle 
of  accommodation.  When  the  ciliary  muscle  is  relaxed,  the 
lens  is  kept  flattened  by  the  pull  of  the  choroid  on  the  liga- 
ment; when  the  muscle  contracts,  this  pull  is  eased  off  (or 
slacked)  and  the  lens  becomes  more  convex.  The  entire 
operation  is  known  as  accommodation,  and  we  may  now 
inquire  what  part  accommodation  plays  in  vision. 

8.  The  formation  of  an  image  by  a  lens.     The  eye  is  a 
camera,  in  that  it  forms  on  the  retina  an  image  of  objects 


SENSE  OKGANS  AND  SENSATIONS  245 

in  front  of  the  cornea;  and  it  is  the  first  essential  of  clear 
vision,  just  as  it  is  the  first  essential  of  photography,  that 
this  image  be  sharp,  or  at  least  distinct.  A  simple  experi- 
ment will  show  that  clear  vision  of  near  and  of  distant  ob- 
jects cannot  be  had  by  the  eye  at  the  same  time.  Hold  up 
a  pencil  or  a  pen  about  ten  inches  from  the  eye  and  look 
first  at  it  and  then  at  some  object  far  away.  Both  can  be 
seen,  but  only  one  at  a  time  clearly, 
and  often  an  effort  is  required  to  shift 
from  the  far  to  the  near  object. 

The  change  which  occurs  in  the 
eye  in  the  act  of  accommodation  is 
illustrated  in  the  following  experi- 
ment: A  wooden  or  pasteboard  box 
(approximately  8  by  5  by  4  inches) 
is  fitted  with  a  piece  of  ground  glass 
on  one  side  and  provided  with  a  con- 
vex lens  on  the  opposite  side.  This 
is  a  rude  camera,  and  some  object  is 
now  placed  at  such  a  distance  that 
the  lens  forms  an  image  of  it  on  the  FIG.  94.  Diagram  of  the 
ground  glass,  which  is  now  in  focus  mechanism  of  accommoda- 
for  the  object.  If,  later,  the  object  , 

J  The  ciliary  muscle  is  repre- 

be     moved    nearer    to     the    lens,     the     sented  as  three  fibers  passing 

focus  is  changed;  the  image  on  the    obliquely  from  the  sclerotic 

to  the  choroid 

glass  becomes  blurred,  and  m  order 

to  make   it  distinct   it  will   be    found  necessary  to   use   a 

more  convex  lens. 

Essentially  the  same  change  occurs  in  the  eye  in  accom- 
modating for  near  objects :  the  lens  must  be  made  more  con- 
vex; and  this,  it  will  be  remembered,  involves  work  on  the 
part  of  the  muscle  of  accommodation  (see  p.  244).  We  can 
thus  understand  why,  in  general,  it  is  too  much  of  "near 
work,"  and  especially  near  work  necessitating  very  distinct 
vision,  that  tires  the  eye.  The  ideal  condition  of  the  eye, 


246 


THE  HUMAN  MECHANISM 


regarded  merely  as  a  camera,  is  that  in  which  distant  objects 
are  focused  on  the  retina  when  the  muscle  of  accommoda- 
tion is  completely  relaxed  and  the  lens  is  thus  flattened  to 
its  utmost  by  the  elastic  pull  of  the  choroid  coat  (p.  243), 
for  iii  this  case  the  eye  is  rested  by  looking  at  distant  ob- 
jects, and  works  only  when  looking  at  near  objects.  Such 
an  eye  is  known  as  an  emmetropic  eye  (Fig.  97,  E). 

Unfortunately,  not  all  eyes  meet  this  requirement.  The 
eyeball  may  be  either  too  short  or  too  long  ;  so  that,  with 
the  muscle  of  accommodation  relaxed,  the  position  of  the 


.-A. 


^"""_'I'--  -;-''":J-- 

; 

~";i'-*.—  . 

"~-~»»^    >>kx 

*—  ..^  * 

V 

FIG.  95.    Action  of  a  convex  lens  in  bringing  to  a  focus  the  rays  of  light 

diverging  from  a  single  point 
The  rays  from  A  are  focused  at  a ;  those  from  B,  at  6 

perfect  focus  for  distant  objects  is  either  before  or  behind 
the  retina;  the  eye  no  longer  sees  distant  objects  distinctly 
when  it  is  at  rest,  because  then  the  retinal  image  is  blurred. 
To  understand  more  fully  the  undesirable  consequences  of 
this  condition,  we  must  learn  how  convex  lenses  produce 
images  of  objects. 

9.  The  action  of  a  convex  lens  on  rays  of  light.  The  rays 
of  light  diverging  from  a  single  point  and  entering  a  convex 
lens  are  bent  so  that  all  come  together  again  in  a  point  be- 
hind the  lens,  or,  as  it  is  said,  are  brought  to  a  focus.  This 
is  shown  in  Fig.  95,  as  is  also  the  fact  that  rays  of  light 
diverging  from  more  distant  points  come  to  a  focus  behind 
the  lens  sooner  than  those  diverging  from  nearer  points. 


SENSE  ORGANS  AND  SENSATIONS  247 

Now  a  lens  forms  an  image  of  an  object  because  all  the 
rays  of  light  from  each  point  of  the  object  are  focused  in 
corresponding  points  behind  the  lens.  This  is  shown  in 
Fig.  96,  where  all  the  rays  diverging  from  1  are  focused 
at  1',  all  those  from  2  at  2',  and  those  from  intermediate 
points  of  the  object  at  intermediate  points  of  the  image. 

If  the  rays  from  each  point  meet  in  front  of  the  retina  and 
then  diverge  before  reaching  the  retina,  the  retinal  image  is 
blurred;  and  the  image  is  also  blurred  if  the  retina  is  so 


ch 


ch 


FIG.  96.   Diagram  showing  the  formation  of  an  image  on  the  retina 

Jf,  2,  the  object ;  1',  2' ,  the  image  of  the  object ;  c,  cornea ;  i,  iris ;  I,  lens ;  v,  vitreous 
humor;  w,  sclerotic;  ch,  choroid;  o.n.,  optic  nerve 

near  the  lens  that  the  rays  from  each  point  have  not  yet 
come  to  a  focus.  The  more  convex  the  lens,  the  more  will 
the  rays  of  light  be  bent;  consequently  we  use  the  muscle 
of  accommodation  (which  makes  the  lens  more  convex)  to 
get  clear  images  of  near  objects  (see  Fig.  95). 

10.  Myopia,  hypermetropia,  and  presbyopia.  In  the  em- 
metropic  eye  (Fig.  97,  E)  the  distance  between  the  retina 
and  the  lens  is  such  that  light  from  distant  points  comes 
to  a  focus  on  the  retina  without  any  active  muscular  ac- 
commodation; to  see  near  objects  the  lens  is  made  more 
convex. 


248 


THE  HUMAN  MECHANISM 


When  the  retina  is  so  far  away  from  the  lens  that,  with 
the  muscle  of  accommodation  completely  relaxed  and  there- 
fore the  lens  flattened  to  its  utmost,  light  from  distant 
points  comes  to  a  focus  in  front  of  the  retina,  the  retinal 
image  is  blurred,  and  it  is  impossible  for  such  an  eye  to  see 
distant  objects  clearly.  To  correct  such  vision  it  would  be 

necessary  to  make  the 
lens  still  less  convex, 
and  this  the  eye  is  un- 
able to  do.  (Why?) 
Such  an  eye  is  known 
as  myopic,  or  near- 
sighted, and  its  defect 
must  be  corrected  by 
the  use  of  concave 
glasses,  which  act  as 
if  the  lens  were  made 
flatter,  and  so  throw 
the  focus  farther  back 
upon  the  retina.  A 
myopic  eye  generally 
has  clear  sight  for 
very  near  objects  be- 
cause, as  stated  above, 


Course  of  the  rays  of  light  from  a 
distant  point 

Through  the  emmetropic(^),  the  myopic  (M),  and 
the  hypermetropic  (H)  eye,  the  muscle  of  accom- 
modation being  relaxed.  (The  rays  diverging 
from  a  distant  point  would  enter  the  eye  practi- 
cally parallel) 


FIG.  97. 

the  nearer  the  object 
the  farther  back  is  the 
image  formed. 

On  the  other  hand, 
the  eyeball  may  be 
too  short,  fore  and  aft  (Fig.  97,  ^T),  so  that,  when  the  ciliary 
muscle  is  relaxed,  light  from  distant  points  has  not  yet  been 
brought  to  a  focus  when  it  reaches  the  retina  (Jiypermetropia). 
Such  an  eye  must  accommodate  not  only  for  near  but  also 
for  distant  objects,  and  its  muscle  of  accommodation  can 
never  rest  so  long  as  the  eye  is  being  used.  Moreover,  to  see 


SENSE  ORGANS  AND  SENSATIONS  249 

near  objects  the  ciliary  muscle  must  work  much  harder 
than  in  the  normal  eye,  and  it  often  happens  that,  even 
with  its  utmost  effort,  the  rays  are  not  sufficiently  bent  to 
focus  them  on  the  retina ;  so  that  a  book,  for  example,  must 
be  held  at  arm's  length  to  be  read.  Persons  having  such 
eyes  form  one  class  of  those  said  to  be  "  far-sighted,"  and 
their  trouble  can  be  corrected  by  the  use  of  convex  glasses. 

As  old  age  ap- 
proaches, changes  oc- 
cur in  the  lens;  in 
consequence,  it  no 
longer  becomes  as 
convex  as  formerly 
in  response  to  the  ac- 
tion of  the  muscle 
of  accommodation 
(presbyopia,  from 
irplffflvt,  "  old  "). 
Some,  though  not 
all,  results  of  this 
condition  resemble 

those     of    hyperme- 

i          ,  i  FIG.  98.   A  test  for  astigmatism 

tropia;  but  the  two 

differ  in  cause.  Hypermetropia  is  due  to  shortness  of  eyeball ; 
presbyopia,  to  failure  of  accommodation. 

11.  Astigmatism.  We  have  thus  far  been  dealing  with 
those  optical  imperfections  due  to  improper  distance  between 
the  lens  and  the  retina.  Another  and  frequently  more  seri- 
ous trouble,  known  as  astigmatism,  results  when  the  cur- 
vature of  the  cornea  (and  sometimes  of  the  lens)  is  not 
perfectly  regular;  that  is,  when  these  surfaces  are  not  seg- 
ments of  perfect  spheres,  but  resemble  in  curvature  the 
side  of  a  lemon.  In  this  case  the  rays  of  light  from  a  point 
are  not  brought  to  a  focus  again  in  a  point  behind  the 
lens;  and  remembering  the  importance  of  sharp  focusing  in 


250  THE  HUMAN  MECHANISM 

securing  distinct  retinal  images,  the  student  will  see  that  this 
defect  must  seriously  interfere  with  clear  vision.  The  optics 
of  astigmatism  are  too  complicated  to  be  explained  in  an 
elementary  work,  but  the  defect  reveals  itself  generally  in 
an  inability  to  see  with  equal  clearness  lines  running  in  dif- 
ferent directions.  Thus  some  of  the  lines  in  Fig.  98  will  be 
sharply  defined  and  black  while  one  is  looking  with  one 
eye  at  the  white  center,  and  others  will  be  blurred  and 
lighter  in  color. 

Astigmatism  is  of  special  importance  in  reading,  because 
the  lines  of  printed  letters  run  in  different  directions.  The 
effort  to  see  clearly  the  printed  page  is  often  severe,  and 
results  in  headaches  and  other  general  disturbances  of  health, 
the  true  cause  of  which  is  often  unsuspected.  The  trouble 
m&y  usually  be  corrected  by  the  use  of  so-called  "  cylindri- 
cal "  glasses ;  that  is,  glasses  which  compensate  the  defects 
of  curvature  in  lens  and  cornea. 

12.  Accommodation  and  "near"  work.  The  above-described 
defects  of  the  eye  as  an  optical  instrument  may  usually  be 
successfully  corrected  by  the  use  of  proper  glasses,  which 
should,  generally  speaking,  be  prescribed  by  a  good  oculist 
and  not  by  an  optician.  Glasses  may  be  used  for  various 
reasons  —  as  a  matter  of  convenience,  as  where  a  person 
with  slight  myopia  wears  them  merely  to  see  distant  objects 
clearly;  or  of  necessity,  as  when  the  myopia  is  more  pro- 
nounced; or  they  may  serve  the  much  more  important  pur- 
pose of  relieving  the  muscle  of  accommodation  of  undue 
work  in  reading  or  sewing,  and  thus  of  avoiding  "  eye- 
strain."  A  hypermetropic  eye  should  always  be  provided 
with  glasses,  since  otherwise  its  muscle  of  accommodation 
cannot  be  rested  by  looking  at  distant  objects.  But  since  it 
is  near  work  which  requires  the  greatest  effort  of  accom- 
modation, it  is  in  reading,  writing,  drawing,  sewing,  etc. 
that  the  eyestrain  is  apt  to  be  greatest.  As  this  kind  of 
work  is  constantly  increasing  in  modern  life,  the  need  for 


SENSE  ORGANS  AND   SENSATIONS  251 

the  complete  correction  of  such  defects  becomes  more  and 
more  necessary.  Those  whose  occupations  require  long- 
continued  use  of  the  eyes  should  see  to  it  that  these 
precious  organs  are  used  only  under  the  most  favorable 
conditions  and  that  all  strain  is  as  far  as  possible  relieved. 
13.  Accommodation  involves  nervous  as  well  as  muscular 
work ;  the  importance  of  sharp  contrast.  The  work  of  the 
muscle  of  accommodation  is  controlled  by  the  nervous  sys- 
tem, and  accurate  accommodation  involves  an  unusually  high 
degree  of  nervous  coordination.  The  strain  thus  imposed 
may  be  lessened  not  only  by  the  use  of  proper  lenses  and 
by  giving  the  mechanism  of  accommodation  periods  of  rest 
(by  looking  for  a  time  at  distant  objects)  but  also  by  using 
the  eyes  in  near  work  under  the  most  favorable  conditions. 
Perhaps  the  most  important  principle  involved  here  is  to 
secure  the  greatest  possible  contrast  between  the  light  and 
dark  parts  of  objects  at  which  we  are  looking.  When  the 
contrast  is  marked,  the  objects  can  be  seen  easily  and  recog- 
nized even  though  the  accommodation  is  not  absolutely  per- 
fect. When,  on  the  other  hand,  the  contrast  is  not  great, 
very  accurate  accommodation  is  necessary.  Important  means 
of  securing  the  maximum  contrast  are  the  following: 

1.  The  avoidance  of  too  little  and  of  too  great  illumination 
of  the  object.    Let  the  student  examine  any  printed  page  with 
different  degrees  of  illumination.    The  contrast  of  white  and 
black  will  be  poor  in   dim  and  in  very  bright   lights,  and 
greatest  with  a  certain  moderate  illumination.    Hence  read- 
ing in  twilight  or  with  sunlight  falling  directly  on  the  page 
means  greater  eyestrain. 

2.  The  avoidance  of  a  flickering   light.    A  steady  light  — 
one  free  from  nicker  —  is  of  the  highest  importance  for  near 
work.    In  this  respect  a  good  kerosene  lamp  (student's  lamp 
or  Rochester  burner)  is  perhaps  the  best  of  all  lights  for 
reading,   provided   the   heat   which   it  gives   off  is  not  too 
great.    Electric  lights  are  good  if  steady,  but  too  frequently 


252  THE  HUMAN  MECHANISM 

they  are  not.  Gas  from  an  ordinary  fishtail  burner  is  one 
of  the  poorest  lights  for  reading  and  sewing.  The  flicker  of 
gas  lights  may,  however,  be  largely  avoided  by  the  use  of 
mantles. 

3.  If  the  printed  matter  is  not  held  steady,  the  effort  of 
accommodation  becomes  much  more  difficult.    Consequently 
it  is  in  general  a  bad  thing  to  read,  and  especially  to  read 
fine    or   poorly   printed    matter,    on    any    but    the    steadiest 
railroad  train. 

4.  The  use  of  very  fine  type  should  be  reduced  to  a  mini- 
mum.   When  such  printed  matter  is  held  at  the  ordinary 
distance   of   eighteen    inches   from    the   eye,   very   accurate 
accommodation    is    needed,   and    this,   we    have    just    seen, 
involves  nervous  strain ;  if  it  is  held  closer  to  the  eye  (so  as 
to  make  a  larger  image  on  the  retina)  the  lens  must  be  made 
much  more   convex  to  focus  it,  and  this   means  excessive 
work  on  the  part  of  the  muscle  of  accommodation.    This  is 
very  undesirable,  and  especially  so  in  youth,  since  then  the 
tissues  of  the  eye  are  more  plastic,  and  excessive  strain  of 
the  muscle  of  accommodation,  pulling  as  it  does  on  the  scle- 
rotic and  the  choroid  coats,  may  lead  to  permanent  deforma 
tion  of  the  curved  surfaces.    The  marked  increase  of  myopia 
within  the  past  forty  or  fifty  years  is  generally  explained  in 
this  way. 

5.  Highly  calendered  paper  objectionable.    Closely  connected 
with  the  size  of  the  type  is  the  character  of  the  paper  on 
which  it  is  printed.    This  should  be  as  dull  as  possible  in 
order  to  avoid  the  confusing  effect  of  a  glossy  surface.    The 
use  of  highly  calendered  paper  in  many  books  and  serial 
publications,  because   such  paper  lends  itself  more  readily 
to  the  reproduction  of  pictures  in  half  tone,  is  a  sacrifice 
of  hygienic  considerations  to  cheapness. 

14.  Visual  sensations.  We  have  shown  (p.  241)  that  the 
sensation  of  sight  does  not  develop  in  the  eye,  but  in  the 
brain,  as  the  result  of  nervous  impulses  sent  thither  over 


SENSE  ORGANS  AND  SENSATIONS  253 

the  fibers  of  the  optic  nerve  from  the  retina.  Just  how  the 
light  falling  upon  the  retina  originates  these  impulses  cannot 
be  discussed  here ;  suffice  it  to  say  that  the  character  of  the 
impulse  differs  according  to  the  color  of  the  light1  stimulat- 
ing the  retina ;  the  lens  focuses  upon  the  retina  a  flat,  colored 
picture  of  the  objects  at  which  it  is  looking,  just  as  a  pho- 
tographic camera  does,  or  as  the  painter  represents  a  scene 
on  canvas.  One  part  of  the  retina  is  thus  stimulated  by  light 
of  one  color,  and  another  part  by  light  of  another  color  or 
by  another  shade  of  the  same  color ;  and  the  different  kinds 
of  impulses  started  in  the  fibers  of  the  optic  nerve  ultimately, 
upon  their  arrival  in  the  brain,  excite  in  consciousness  what 
we  know  as  visual  sensations.  The  sensations  which  we  get 
from  the  retina  are  therefore  primarily  sensations  of  color. 

15.  Visual  judgments.  But  when  we  look  at  an  object  we 
get  more  than  mere  sensations  of  color.  The  world  does 
not  appear  to  us  as  a  flat  surface,  of  different  colors,  like 
the  painter's  canvas.  When  we  look  at  the  wall  of  a  room 
we  know  that  it  is  a  flat  surface,  and  when  we  look  at  a  box 
we  know  that  it  has  not  only  length  and  breadth  but  also 
thickness.  If  we  were  dependent  entirely  upon  the  retinal 
image  for  our  idea  of  the  box,  it  would  look  as  flat  as  the 
wall ;  that  it  does  not  appear  so  is  because  we  receive  other 
information  about  the  box  than  that  which  comes  from  the 
retina.  We  have  to  accommodate  the  lens  differently  for 
the  near  and  the  far  edges,  and  we  have  learned  by  experi- 
ence that  this  necessity  indicates  depth,  or  different  distances 
of  different  parts  of  the  object.  Again,  we  see  the  box  with 
both  eyes,  and  the  images  formed  on  the  two  retinas  are  not 
exactly  the  same.  One  eye  sees  more  of  one  side,  the  other 
eye  more  of  another  side ;  and  while  we  are  not  conscious 
of  this  fact,  we  have  really  learned  by  experience  and  by 
the  actual  handling  of  objects  that  this  slight  difference  in 

1  In  this  and  the  following  paragraphs  white,  black,  and  gray  are 
regarded  as  colors. 


254 


THE  HUMAN  MECHANISM 


sensations  from  the  two  eyes  are  produced  only  by  solid 
objects.  Again,  when  we  look  at  any  point  on  the  near  edge 
of  a  box  the  two  eyes  are  converged  by  their  muscles  to  a 
greater  extent  than  when  we  look  at  a  point  on  the  far  edge, 
and  we  have  learned  that  these  different  pulls  of  muscles  and 
positions  of  eyeballs  indicate  that  the  object  is  not  flat,  but 
has  depth.  The  importance  of  binocular  vision  in  the  estima- 
tion of  depth  or  distance  from  the  eye  is  most  strikingly 
illustrated  by  attempting,  with  one  eye  closed,  to  bring 
together  the  points  of  two  pencils  held  in  the  hands  and 
moved  from  side  to  side  at  arm's  length. 

Consequently  when  we  look  at  anything  we  get  a  number 
of  sensations ;  from  the  retina,  those  of  color  and  the  posi- 
tion of  the  color  spots  with 
reference  to  one  another ;  from 
the  muscular  efforts  of  accom- 
modation and  of  convergence 
of  the  eyeballs,  those  which 
reveal  the  property  of  depth 
in  what  we  see.  And  from 
all  of  these,  fused  together 
and  interpreted  in  the  light 
of  experience,  we  construct  a 


,1 


'  *  *  < 


' 


> 


•      * 


! 


I 


!  s 

>  ^ 


5 


:  : 


< 


i 


FIG.  99 


visual  judgment  of  the  nature 

of  the  object. 

16.  Optical  illusions.   That 

our   vision  is  essentially  the 

result  of  unconscious  judgments  is  strikingly  shown  by  the 
fact  that  these  sometimes  deceive  us.  Thus  the  parallel 
vertical  lines  in  Fig.  99,  when  crossed  by  the  oblique  lines,  seem 
to  be  inclined  toward  each  other.  The  retinal  images  of  the 
lines  are  parallel,  and  we  falsely  judge  them  inclined,  this 
error  of  judgment  arising  from  the  presence  of  the  oblique 
lines.  In  other  words,  our  final  idea  of  the  lines  does  not 
correspond  to  their  image  on  the  retina. 


SENSE  ORGANS  AND  SENSATIONS  255 

Many  other  examples  might  be  given  showing  that  our 
visual  idea  of  the  world  around  us  is  not  a  simple  sensa- 
tion or  impression,  but  an  unconscious  inference,  judgment, 
or  conclusion  built  up  from  a  number  of  simple  sensations 
taken  separately  or  blended  together  and  compounded  with 
results  of  lifelong  experience.  In  looking  at  a  piece  of  fine 
silk  or  cloth  we  seldom  stop  to  think  that  its  tissue  may  be 
resolved  into  many  simple  component  threads;  and  in  quite 
the  same  way  we  fail  to  realize  that  even  our  quickly  formed 
judgments  of  the  size,  distance,  form,  or  color  of  objects  are 
likewise  tissues  woven  out  of  many  threads,  most  of  which  we 
have  been  slowly  and  laboriously  spinning  since  childhood's 
days  in  the  hidden  factory  of  individual  experience. 

17.  Sound   and    hearing.     When    the   string   of   a   violin, 
piano,  or  harp  "  sounds,"  one  can  observe  that  it  is  in  rapid 
vibration ;  and  the  same  thing  is  true  of  all  sounding  bodies. 
These  vibrations  are  imparted  to  the   air,   water,   or  other 
surrounding   medium,   and    through   this    medium  they  are 
transmitted  as  waves  of  sound.    It  is  these  waves,  or  vibra- 
tions, which,  on  entering  the  ear,  excite  the  sensation  of  sound. 
The  more  rapid  the  vibrations,  the  higher  is  the  pitch  of  the 
note ;  and  the  greater  their  amplitude,  the  louder  the  sound.  . 

The  ear  is  an  organ  specially  adapted  to  receive  these  vibra- 
tions of  air  and  to  transform  them  into  nervous  impulses. 
It  is  subdivided  by  anatomists  into  the  outer  ear,  the  middle 
ear,  and  the  inner  ear. 

18.  The  outer  ear.    The  outer  ear  consists  of  the  expanded 
pinna  (or  that  part  which  we  commonly  call  "  the  ear  ")  and 
a  tube  along  which  the  vibrations  of  sound  pass  inward  to 
the  tympanic  membrane,  or  drum.     Glands  along  this  canal 
secrete  wax  which  guards  the  approach  to  the  drum.    It  is  a 
bad  habit  to  pick  at  this  wax,  and  especially  to  dig  into  the 
ear  with  any  pointed  instrument,  for  there  is  always  danger 
of  perforating  the  drum.    If  trouble  is  suspected,  a  physician 
should  be  consulted. 


256 


THE  HUMAN  MECHANISM 


19.  The  middle  ear ;  the  Eustachian  tube.  The  tympanic 
membrane  separates  the  outer  from  the  middle  ear,  or  tym- 
panum, a  small  cavity  lying  in  the  temporal  bone  of  the 
skull  and  communicating  with  the  throat  or  pharynx  by 
means  of  the  Eustachian  tube.  The  air  which  it  contains 
is  consequently  under  the  same  pressure  as  that  of  the 


FIG.  100.   Diagram  of  the  ear 

A,  the  auditory  canal,  leading  to  the  tympanic  membrane  B;   C,  cavity  of  the 
tympanum,  communicating  by  the  Eustachian  tube  with  the  pharynx  D  \  E,  semi- 
circular canals ;  F,  cochlea ;  G,  auditory  nerve 

atmosphere  without,  and  the  tympanic  membrane  is  not 
normally  bulged  inward  or  outward  by  inequality  of  pres- 
sure on  its  two  sides.  The  opening  of  the  Eustachian  tube 
into  the  pharynx  is,  however,  closed  except  when  one  swal- 
lows, and  hence  swallowing  often  relieves  the  drum  from 
undue  pressure  of  air  in  the  middle  ear. 

The  cavity  of  the  tympanum  also  communicates  with  a 
network  of  spaces,  or  sinuses,  in  the  temporal  bone.    Because 


SENSE  ORGANS  AND  SENSATIONS 


257 


of  these  connections  of  the  middle  ear  with  the  throat,  on 
the  one  hand,  and  with  the  temporal  sinuses  on  the  other, 
inflammatory  processes  in. the  nose  and  throat  during  a  cold 
sometimes  extend  into  the  Eustachian  tube,  the  tympanum, 
and  even  into  the  temporal  sinuses,  causing  serious  trouble 
and  occasionally  deafness. 

Passing  directly  across  the  tympanum,  from  the  drum  on 
its  outer  side  to  the  cochlea  on  its  inner  side,  is  a  chain  of 
three  very  small  bones,  the  ear 
ossicles  (hammer,  anvil,  and  stir- 
rup). These  bones  are  bound 
together  and  attached  to  the 
walls  of  the  tympanum  by  liga- 
ments, and  are  so  arranged  that 
when  sound  waves  set  the  tym- 
panic membrane  in  vibration 
this  motion  is  transmitted  by 
the  ossicles  to  a  portion  of  the 
inner  ear  known  as  the  cochlea. 

20.  The  inner  ear.  The  struc- 
tures, of  the  inner  ear  lie  in 
the  temporal  bone,  on  the  side 
of  the  tympanum  opposite  the 
drum.  They  consist  of  a  system 
of  small  bony  spaces  and  tubes, 
the  ~bony  labyrinth,  within  which  lies  a  corresponding  membra- 
nous labyrinth.  Forming  part  of  the  lining  of  the  membranous 
labyrinth  are  very  sensitive  cells,  and  between  these  cells 
are  the  endings  of  the  nerve  fibers  which  connect  the  ear 
with  the  brain.  The  cells  of  the  inner  ear  are  sensitive  to 
the  vibrations  which  have  been  transmitted  across  the  tym- 
panum by  the  ossicles,  just  as  the  retina  is  sensitive  to 
light;  and  as  the  retina  is  the  origin  of  the  fibers  of  the 
optic  nerve,  so  the  inner  ear  is  the  origin  of  those  of  the 
auditory  nerve. 


FIG.  101.    The  bony  labyrinth,  its 

actual   size   being  shown  in   the 

smaller  figure 

B,  (7,  D,  the  semicircular  canals; 
A,  the  oval  window,  by  means  of 
which  the  vibrations  of  the  stirrup 
bone  are  transmitted  to  the  cochlea ; 
E,  F,  G,  the  whorls  of  the  cochlea. 
Cf .  Fig.  102 


258 


THE  HUMAN  MECHANISM 


Vestibule  with  Openings 
of  Semicircular  Canals 


Scala  Vestibuli 


Cochlea 


Eustachian  Tube*^-. 


FIG.  102.    Diagrammatic  representation  of  the 

membranous  labyrinth  of  the  cochlea  in  relation 

to  the  structures  shown  in  Figs.  100  and  101 

The  scala  vestibuli  and  scala  tympani  are  the  two 

portions    of    the    bony  cochlea  which   inclose    the 

membranous  cochlea 


21.  Taste  and  smell.    The  end  organs  of  taste  are  small 
rounded  eminences,  or  papillce,  on  the  dorsal  surface  of  the 
tongue,  and  from  these  the  fibers  of  the  nerves  of  taste  pass 
to  the  brain.    The  end  organs  of  the  nerve  of  smell  are  situ- 
ated in  the  upper  portion  of  the  nasal  cavity  and  consist 
of   delicate   cells  very  sensitive    to   the  presence   of   odors. 
Sensations  of  taste  are  frequently  confounded  with  those  of 
smell.    An  onion,  for  example,  has  little  or  no  taste,  as  can 

be  shown  by  placing 
a  bit  on  the  tongue 
when  one  is  holding 
the  breath ;  none  of 
the  flavor  of  the 
onion  is  perceived. 
On  the  other  hand, 
sour,  sweet,  bitter., 
Scala'Tympani  and  salt  are  true 

sensations  of  taste. 
This  unconscious 
blending  of  tastes 
with  odors  in  form- 
ing our  ideas  of  the 
nature  of  objects  re- 
calls the  formation  of  visual  judgments  by  the  combination 
of  retinal  sensations  with  those  aroused  by  the  muscular 
act  of  converging  the  eyeballs. 

22.  Cutaneous  sensations.    The  skin  is  the  place  of  origin 
of  at  least  three  sensations  —  touch,  cold,  and  warmth.    These 
sensations  are  distinct,  as  is  shown  by  the  observation  that 
on  certain  points  of  the  skin  some*of  them  may  be  felt,  but 
not  others.     This  fact  is  usually  interpreted   to  mean  that 
each   sensation   has   its   own   set   of   end   organs   and   nerve 
fibers.    Especially  striking  is  the  fact  that  warmth  and  cold 
are  not  felt  by  the  same  spot  of  skin,  which  seems  to  prove 
conclusively  that  they  are  separate  sensations. 


SENSE  OKGANS  AND  SENSATIONS 


259 


The  afferent  nerves  of  cold  and  warmth  not  only  carry 
into  the  brain  those  impulses  which  give  rise  to  the  corre- 
sponding sensations  but  also  serve  as  one  important  means 
of  stimulating  the  reflexes  which  help  to  regulate  heat 
production  and  heat  output  (see  Chap.  XII). 

23.  The  sense  of  position.  The  expression  "the  five  senses" 
has  become  proverbial,  and  comes  from  the  time  when  sight, 
hearing,  taste,  smell,  and  touch  were  the  recognized  special 
senses.  To-day,  however,  we  must  add  to 
these  not  only  warmth  and  cold  but  still 
others,  most  conspicuous  among  which  is  the 
sense  of  position.  When  the  eyes  are  closed 
we  are  aware  of  the  position  of  the  various 
parts  of  the  body.  We  know  whether  the 
arm  is  bent  at  the  elbow  or  straight ;  whether 
the  head  is  looking  forward  or  is  turned  to 
one  side  or  the  other.  And  while  we  are 

aware    of   these   things,   partly   from   tactile 

.       8  '    F  .     J       . .  FIG.  103.    A  tac- 

sensations,  there  is  conclusive  evidence  that    tiie  corpuscle  in 

afferent  impulses  from  the  muscles,  tendons,     one  of  the  papil- 

and  joints   also  play   an   important  part   in    lse  of  t?e  dermis ] 

,  ,  an  end  organ  of 

the  result  the  sense  of  touch 

When  one  is  blindfolded  and  lies  flat  on  a 
revolving  table  which  can  be  turned  noiselessly  in  one  direc- 
tion or  the  other,  the  subject  of  experiment  can  form  fairly 
correct  judgments  as  to  the  angle  and  direction  through 
which  the  table  is  turned.  Here  there  is  no  change  of  char- 
acter either  in  the  tactile  impulses  or  in  those  from  the  mus- 
cles, tendons,  and  joints,  for  the  subject  of  experiment  lies 
still  and  is  only  passively  moved.  It  is  believed  that  in  this 
case  the  sensations  in  question  come  from  the  movements  of 
the  lymph  in  portions  of  the  inner  ear.  One  part  of  this, 
the  cochlea,  is  undoubtedly  concerned  with  the  perception  of 
sound ;  but  another  part,  the  three  semicircular  canals,  are 
now  believed  to  be  end  organs  of  this  sense  of  position. 


260  THE  HUMAN  MECHANISM 

The  impulses  which  make  us  aware  of  the  position  of  parts 
of  our  bodies  also  play  a  very  important  role  in  reflexly 
guiding  our  movements.  Upon  this  we  shall  dwell  at  greater 
length  in  subsequent  chapters  (see  especially  Chap.  XV). 

24.  Sensations  of  pain.  Most  organs  of  the  body  may  also 
give  rise  to  impulses  which,  on  their  arrival  in  the  brain, 
cause  sensations  of  pain.  It  is  still,  perhaps,  an  open  ques- 
tion whether  this  sensation,  like  sight,  smell,  and  hearing,  is 
aroused  by  its  own  mechanism  of  end  organs  and  afferent 
nerves  or  whether  it  is  called  forth  by  the  excessive  stimu- 
lation of  the  nerves  of  the  other  senses,  but  for  the  discus- 
sion of  this  question  the  reader  must  consult  more  advanced 
works  on  physiology. 

Pain  is  a  useful  danger  signal,  since  it  effectively  calls 
attention  to  abnormal  conditions  and  incites  us  to  the  adop- 
tion of  active  remedial  measures.  Remedies,  however,  should 
not  be  confined  to  the  abolition  of  unpleasant  sensations,  but 
should  be  directed  to  the  removal  of  their  cause.  A  tooth- 
ache from  a  decaying  tooth  may  often  be  stopped,  for  a  time 
at  least,  by  the  use  of  chloroform  or  other  anesthetic  drugs, 
but  the  drug  only  stops  the  pain ;  it  does  not  check  the 
progress  of  decay  or  repair  the  damage.  Again,  a  bronchial 
cough  may  be  unpleasant  and  even  painful,  but  we  should 
not  rest  content  with  the  use  of  some  drug  or  cough  medicine 
which  merely  lessens  the  irritability  of  the  inflamed  surface 
of  the  air  passages,  and  so,  perhaps,  stops  the  cough  without 
curing  the  disease. 

Pain  is  a  warning  that  some  abnormal  condition  needs 
attention.  Sometimes  that  attention  may  be  supplied  by  the 
sufferer  himself,  or  by  his  friends,  but  often  skilled  medical 
advice  is  needed.  Too  frequently,  for  the  sake  of  economy 
or  from  feelings  of  modesty,  or  even  because  of  an  unwilling- 
ness to  acknowledge  illness  either  to  the  world  or  to  one's  self, 
the  mistake  is  made  of  postponing  the  visit  to  the  physician, 
the  patient  meanwhile  bearing  discomfort  and  perhaps  actual 


SENSE  ORGANS  AND  SENSATIONS  261 

suffering  in  the  hope  that  he  will  soon  be  better  and  that  the 
trouble  will  "  cure  itself."  Sometimes,  of  course,  it  does  cure 
itself ;  but  sometimes  it  does  not ;  and  remediable  disease 
has  too  frequently  been  allowed  to  run  on  in  this  way  until 
some  vital  spot  is  attacked  or  the  trouble  has  become  too 
grave  for  medical  skill  to  overcome.  Many  diseases,  like  a 
fire,  may  be  extinguished  at  the  start,  but  if  not  attended  to, 
grow  rapidly  into  a  conflagration  beyond  control.  Pain  is 
one  of  the  most  trustworthy  warnings  that  attention  to  the 
mechanism  itself  or  to  our  operation  of  it  is  necessary;  and 
we  have  no  right,  either  for  our  own  sake  or  that  of  our 
friends,  to  neglect  its  warnings.  While  there  are  times  when 
it  is  an  act  of  heroism  to  endure  suffering  and  to  keep  the 
knowledge  of  it  to  one's  self,  there  are  other  times  when  to 
do  this  is  not  only  foolish  but  wrong. 

25.  Hunger  and  thirst.  No  account  of  the  physiology  of 
sensations  would  be  complete  without  some  reference  to 
those  very  common  experiences  of  life  —  hunger  and  thirst. 
We  have  already  spoken  of  them  as  sensations  which  are 
referred  to  the  body  and  never  to  external  objects,  thirst 
usually  being  referred  to  the  mouth  and  throat,  and  hunger 
frequently  to  the  stomach ;  but  hunger  and  even  thirst  may 
sometimes  affect  us  as  sensations  coming  from  the  body  as 
a  whole,  in  which  case  they  are  usually  indistinguishable 
from  certain  forms  of  general  fatigue. 

Hunger  is  excited  by  automatic  rhythmic  contractions  of 
the  musculature  of  the  cardiac  end  of  the  stomach.  The 
stomach,  like  the  heart,  executes  rhythmic  contractions,  and 
we  may  speak  of  the  "  beat "  of  the  stomach  just  as  we 
speak  of  the  "  beat "  of  the  heart,  although  each  stomach 
contraction  is  much  slower  than  those  of  the  heart.  When 
food  is  in  the  stomach,  these  contractions  or  "  beats "  are 
inhibited  in  the  cardiac  end  or  else  are  reduced  to  very  in- 
significant proportions,  and  we  have  the  inactive  condition 
of  this  portion  of  the  stomach  described  in  Chapter  VIII ; 


262  THE  HUMAN  MECHANISM 

but  when  the  cardiac  pouch  is  again  empty,  the  inhibiting 
check  is  removed  and  the  automatic  "  beats "  become  quite 
powerful.  These  contractions  start  impulses  up  the  sensory 
nerves  of  the  stomach,  and  these  impulses  excite  in  our  con- 
sciousness sensations  of  hunger.  Often  the  "  beats "  occur 
in  rhythmic  periods,  a  group  of  strong  contractions  alter- 
nating with  groups  of  weak  contractions  or  even  total 
quiescence.  In  this  case  we  have  the  "  griping "  hunger 
pangs  coincident  with  the  strong  contractions.  In  certain 
abnormal  conditions  the  presence  of  food  in  the  stomach 
fails  to  exert  its  inhibiting  effect  and  we  have  a  continual 
"  gnawing  "  hunger. 

Thirst  is  aroused  by  the  dryness  of  the  mouth  and  throat, 
probably  by  the  reduction  of  the  amount  of  water  in  cells 
and  tissues  of  this  organ. 

Hunger  and  thirst  are  definite  sensations,  as  truly  adapted 
to  guide  us  in  the  choice  of  food  as  sight  is  adapted  to 
picture  to  us  the  world  in  which  we  live.  So  long  as  the 
body  is  normally  occupied  and  healthy  they  may  usually 
be  trusted;  but  there  are  abnormal  conditions  of  sedentary 
life,  in  the  midst  of  a  superabundance  of  tempting  food, 
when  they  become  less  trustworthy,  and  in  some  forms  of 
dyspepsia  the  sensation  of  hunger  is  never  absent,  no  matter 
how  often  one  eats.  In  such  cases  the  very  effort  to  satisfy 
hunger  only  aggravates  disease.  Conditions  of  this  sort 
should  not  prevail  if  proper  attention  be  paid  to  the  general 
hygienic  conduct  of  life.  Broadly  speaking,  appetites,  like 
fire  and  dynamite,  are  good  servants  but  bad  masters. 


CHAPTER  XV 


THE  NERVOUS  SYSTEM 
A.  ITS  ANATOMICAL  BASIS 

In  the  preceding  chapter  we  have  repeatedly  emphasized 
the  fact  that  sensations  of  all  kinds  are  developed  in  the 
brain  from  nervous 
impulses  coming 
from  the  sense  or- 
gans, and  in  a  pre- 
vious chapter  (VII) 
we  have  seen  that 
Avithout  reaching 
the  brain,  or  at 
least  without  af- 
fecting conscious- 
ness, these  affer- 
ent impulses  may 
give  rise  to  reflex 
action.  A  reflex  ac- 
tion or  a  conscious 
sensation,  or  both  a 
reflex  action  and 


FlG-  104-    Tne  human  brain  viewed  from  above. 

The  cerebral  hemispheres  completely  cover  the 

rest  of  the  brain 


a   conscous   sensa- 

tion, may  therefore 

„    -.,   j-  ,1 

trance  of  a  nervous 
impulse  into  the  central  nervous  system,  and  we  have  now 
to  inquire  what  is  known  of  the  mechanism  by  which  these 
results  are  brought  about. 

263 


264 


THE  HUMAN  MECHANISM 


1.  Fundamental  structure  of  the  nervous  system;  the  brain 
of  a  frog.    The  human  spinal  cord  and  brain  are  so  com- 


plicated that  it  is  best  to 


Forebrain 

'  Tweenbrain 
Midbrain 

Hindbrain 


Spinal 
Cord 


EIG.  105.    The  brain  and  spinal  cord  of  the  frog 

On  the  left  is  a  longitudinal,  right-to-left  section, 
showing  the  central  canal  and  the  ventricles  of  the 
brain ;  on  the  right  the  dorsal  view  of  the  brain  and 
cord.  A,  the  c'erebral  hemispheres ;  B,  the  optic  lobes ; 
C,  the  cerebellum;  D,  the  bulb;  E,  the  spinal  cord 


study  first  the  nervous  system 
of  a  simple  verte- 
brate like  the  frog, 
for  the  fundamen- 
tal plan  of  struc- 
ture is  the  same  in 
both.  The  spinal 
cord  is  a  relatively 
thick-walled  tube, 
«  I  p  the  walls  of  which 

J  I  I  are    composed    of 

1  white      and     gray 

matter,  the  minute 
bore,  or  lumen,  of 
the  tube  being 
known  as  the  cen- 
tral canal.  The  ar- 
rangement in  the 
brain  is  similar,  but 
here  the  central 
space  is  no  longer 
a  small  tube  of  even 
bore,  but  consists 
for  the  greater  part 
of  irregular  cavities 
known  as  the  ven- 
tricles of  the  brain, 
while  the  walls 
consist  of  masses 
of  gray  and  white 


matter  varying  in  size,  shape,  and  relation  to  each  other. 

Fig.  105  will  assist  the  student  in  understanding  this  plan 
of  structure.    Anteriorly  the  spinal  cord  is  continued  in  the 


THE  KEKVOUS  SYSTEM  265 

bulb,  *  whose  central  cavity  is  the  fourth  ventricle.  Part  of 
the  dorsal  wall  of  this  ventricle  forms  the  cerebellum,  which 
in  the  frog  is  only  slightly  developed,  but  which  in  higher 
vertebrates  (birds  and  mammals)  becomes  a  large  and  con- 
spicuous organ.  Anteriorly  the  fourth  ventricle  is  connected 
with  the  third  by  a  tube,  the  aqueduct  of  Sylvius.  The  thick 
walls  of  this  aqueduct  contain  various  masses  of  gray  matter 
whose  names  need  not  detain  us ;  the  walls  of  the  third  ven- 
tricle are  similarly  composed  of  large  masses  of  gray  matter 


FIG.  106.    Diagrammatic  median  longitudinal  section  of  a  mammalian  brain 

After  Edinger 

For  convenience  the  cerebrum,  with  its  lateral  ventricle,  is  represented  as  a  single 

organ  in  the  median  plane  instead  of  two  hemispheres  on  either  side  of  this 

plane  and  each  with  its  own  lateral  ventricle.     The   division  into  forebrain, 

'tweenbrain,  midbrain,  and  hindbrain  is  marked  by  the  broken  lines 

scattered  among  the  fibers  of  the  white  matter.  Still  farther 
forward  two  openings  from  the  third  ventricle,  one  on  the 
right  and  one  on  the  left  side,  lead  into  the  large  lateral 
ventricles,  the  nervous  tissue  of  whose  walls  is  the  cerebrum, 
or  the  cerebral  hemispheres.  It  is  convenient  to  divide  the  brain 
into  the  forebrain,  surrounding  the  lateral  ventricles  ;  the 
'tweenbrain,  surrounding  -the  third  ventricle ;  the  midbrain, 
surrounding  the  aqueduct  of  Sylvius ;  and  the  hindbrain, 
surrounding  the  fourth  ventricle. 

1  The  older  term  for  the  bulb  is  the  medulla  oblongata,  to  distinguish 
it  from  the  medulla  spinalis,  or  spinal  cord. 


266 


THE  HUMAN  MECHANISM 


2.  The  brain  of  the  mammal  is  built  on  the  same  funda- 
mental plan  as  that  of  the  frog,  and  differs  from  it  mainly 
in  the  greater  number  of  neurones  and  in  the  complexity 


B 


FIG.  107.   The  base  of  the  human  brain,  showing  the  cranial  nerves 

A,  the  cms  cerebri,  composed  largely  of  nerve  fibers  which  connect  the  hind- 
brain  with  the  'tweenbrain  and  forebrain ;  B,  the  pans  Varolii,  the  anterior  floor 
of  the  fourth  ventricle,  connected  laterally  with  the  cerebellum;   (7,  the  bulb; 
D,  the  cerebellum;   E,  the  spinal  cord 

of  their  connections  with  one  another.  This  results  in 
great  thickening  of  the  ventricular  walls  and  the  formation 
of  a  very  complicated  anatomical  structure.  Mammals  are 
especially  characterized  by  an  enormous  development  of  the 


THE  NEKVOUS  SYSTEM 


267 


cerebral  hemispheres,  which  in  man  grow  to  such  proportions 
upwards  and  backwards  as  to  overhang  and  completely  cover 
the  other  structures  on  the  dorsal  side.  But  even  these  large 
masses  of  nervous  tissue,  no  less  than  the  smaller  cerebrum 
of  the  frog,  are  composed  entirely  of  the  gray  and  white 
matter  forming  the  walls  of  the  lateral  ventricles. 

By  comparing  the  brain  of  a  frog  (Fig.  105)  with  those 
of  the  rabbit,  cat,  and  monkey  (Fig.  166),  and  finally  with 


FIG.  108.    Median  longitudinal  section  of  the  human  brain 

A,  B,  C,  D,  L,  convolutions  of  the  median  surface  of  the  cerebrum ;  E,  F,  the 

cerebellum,  showing  in  the  plane  of  section  the  inner  white  matter  and  the  outer 

gray  matter ;  H,  the  pons  Varolii ;  If,  the  bulb 

the  human  brain  (Figs.  104,  107,  108),  a  fairly  good  idea 
may  be  had  of  the  increasing  complexity  of  the  brain  as  we 
pass  from  the  lower  to  the  higher  animals.  Especially  note- 
worthy is  the  greater  relative  prominence  of  the  cerebrum. 
In  the  frog  this  organ  is  small  and  inconspicuous ;  in  the 
rabbit  it  is  much  larger,  but  its  surface  is  smooth ;  in  the  cat 
there  is  a  further  increase  in  size,  and  the  surface  is  thrown 


268 


THE  HUMAN  MECHANISM 


into   folds,    or   convolutions-,    and   this    increase   in   size   and 
surface  folding  —  carried  yet  farther  in  the  monkey  —  reaches 

its  highest   development 
in  the  human  brain. 

3.  The  cranial  nerves. 
Nerves  enter  the  'tween- 
brain,  midbrain,  and  hind- 
brain  somewhat  as  they 
enter  the  spinal  cord  ; 
and  although  their  sepa- 
ration into  dorsal  and 
ventral  roots  is  not  ob- 
vious, the  neurones  to 
which  their  nerve  fibers 
belong  are  in  all  respects 
analogous  to  the  neu- 
rones of  the  spinal  nerves. 
They  may  serve  as  the 
paths  of  reflexes  (for  ex- 
ample, a  wink  is  a  re- 
flex from  the  optic  or  the 
trigeminal  nerve  to  the 
facial  nerve),  and  their 
relation  to  the  cells  of 
the  cerebrum  and  other 
higher  portions  of  the 

FIG.  109.    A  portion  of  the  gray  matter     bram     *    essentially    the 
(cortex)  of  the  cerebrum  (highly  magni-     same  as  that  of  the  spinal 


fied).  After  Kolliker 


nerves.  Fig.  107  will  give 


Note  the  large   number  of  dendrites.    The      the  pointg  Qf  entrance  or 


axons   are   the   fibers  of  uniform   diameter 

running  lengthwise  of  the  drawing.    One  of 

these  cells  is  shown  in  Fig.  41,  D 


exit  of  these  nerves  from 
the  human  brain. 
4.  Histological  structure  of  the  brain.    Microscopic  study 
of  the  brain  shows  an  aggregation   of  neurones   similar  to 
that  seen  in  the  spinal  cord.    These  neurones  differ  greatly 


THE  KEKVOUS  SYSTEM  269 

in  shape  (see  Chap.  VII,  p.  73),  in  the  number  of  their 
dendrites,  and  in  the  abundance  of  their  connections  with 
other  neurones.  The  regular  arrangement  in  the  cord  of 
central  gray  matter  surrounded  by  white  matter  is  wanting ; 
instead,  masses  of  gray  matter  occur  here  and  there  among 
the  bundles  of  nerve  fibers  of  which  the  white  matter  is 
composed.  In  the  cerebrum  and  cerebellum  the  external 
surface  consists  of  gray  matter  and  is  known  as  the  cortex 
of  the  cerebrum  and  cerebellum  respectively.  These  cortical 
structures  form  the  most  complicated  system  of  nervous 
tissue  in  the  body,  and  the  cerebral  cortex  is  intimately 
concerned  with  the  highest  functions  of  the  brain.  (See 
Figs.  109,  110,  and  111.) 

The  figures  give  some  idea  of  the  variety  and  complexity 
of  the  neurones  of  the  brain.  But  however  different,  at  first 
sight,  the  brain  may  be  from  the  spinal  cord,  the  anatomical 
plan  of  organization  is  the  same  in  both;  the  brain  as  well 
as  the  cord  does  its  work  because  the  connections  of  its  neu- 
rones with  one  another  bring  about  coordinated  action.  The 
secret  of  the  structure  of  the  brain,  as  of  the  cord,  lies  in 
the  nature  of  the  connections  of  its  units,  the  neurones,  one 
with  another. 

B.  THE  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM 

Whenever  through  accident,  disease,  or  otherwise,  some 
portion  of  the  nervous  system  is  destroyed,  functions  depend- 
ent upon  it  are  no  longer  performed,  or  at  least  are  not  per- 
formed normally.  A  very  large  number  of  observations  have 
been  made  upon  both  animals  and  men  in  this  condition,  and 
these  have  made  it  possible  for  us  to  obtain  some  idea  of  the 
part  played  in  normal  life  by  each  part  of  the  brain  and  cord. 
We  shall  attempt  here  to  sketch  only  a  few  of  the  more  im- 
portant outlines  of  the  picture,  which  the  reader  may  com- 
plete by  more  extensive  study  of  physiology  and  psychology. 


270 


THE  HUMAN  MECHANISM 


We  shall  choose  for  study  the  case  of  a  single  animal,  the 
frog,  the  anatomical  structure  of  whose  brain  has  been  given 
in  this  chapter.  The  phenomena  shown  by  the  frog  are, 
however,  as  far  as  we  shall  describe  them,  in  general  true 
of  higher  vertebrate  animals. 

We  shall  therefore  study  (1)  the  behavior  of  a  frog  whose 
brain  has  been  destroyed,  that  is,  a  frog  which  possesses  no 

part  of  its  central 
nervous  system  ex- 
cept the  spinal 
cord ;  (2)  the  be- 
havior of  a  frog 
with  spinal  cord 
and  bulb  intact, 
but  destitute  of 
midbrain,  'tween- 
brain,  and  cere- 
brum ;  (3)  the 
behavior  of  a  frog 
with  spinal  cord, 
bulb,  midbrain,  and 
'tweenbrain,  but 
destitute  of  the 
cerebrum. 

•The  behavior  of 
these  incomplete 
animals  will  each 
be  compared  with  that  of  a  normal  frog,  which,  of  course, 
possesses  a  complete  nervous  system. 

5.  The  behavior  of  a  brainless  frog ;  that  is,  a  frog  which 
possesses  of  its  nervous  system  only  the  spinal  cord.  Such 
a  frog  can  carry  out  only  reflex  actions  of  a  comparatively 
simple  character.  It  lies  flat  upon  its  belly  and,  like  the 
normal  frog,  bends  its  hind  legs  under  its  flank,  but  does 
not  sit  erect  by  supporting  the  head  and  upper  trunk  on  the 


FIG.  110.    Transverse  section  of  a  convolution  of 
the  cerebellum.   After  Ramon  y  Cajal 

The  figure  represents  only  a  few  of  each  kind  of  nerve 

cells  and  nerve  endings.    A,  D,  E,  cells ;  B,  C,  nerve 

endings  (synapses) 


THE  NERVOUS  SYSTEM 


271 


fore  legs.  There  are  no  respiratory  movements;  the  vaso- 
constrictor tone  of  the  blood  vessels  is  impaired  or  absent, 
as  are  also  many  other  of  the  most  important  reflexes. 

But  if  one  leg  be  pulled  gently  backward,  the  animal  will 
bend  it  again  to  its  normal  position  under  the  body.  If  the 
toe  be  pinched,  the  leg  will  suddenly  be  drawn  away;  and 
if  the  skin  of  the  flank  be  irritated  by  a  bit  of  filter  paper 
moistened  with  acid,  the  paper  will  be  kicked  off  by  the  leg 
of  the  same  side. 

These  are  all  pur- 
poseful 1  and  coordi- 
nated actions,  and 
make  upon  the  inex- 
perienced observer 
the  impression  that 
the  frog  is  aware 
of  the  stimulus  and 
acts  intelligently. 
But  the  mere  fact 
that  an  act  is  pur- 
poseful and  coordi- 
nated does  not  show 


FIG.  111.   Section  of  the  cortex  of  the  cerebellum 

(at  right  angles  to  that  shown  in  Fig.  110).  After 

Ramon  y  Cajal 


that  it  is  a  conscious 
act ;  our  movements 
of  respiration,  winking,  coughing,  and  sneezing  are  purpose- 
ful and  coordinated,  but  we  know  well  enough  that  they, 
as  well  as  more  complicated  actions,  may  and  often  do  occur 
in  the  complete  absence  of  consciousness.  One  of  the  first 
lessons  that  the  student  of  animal  behavior  must  learn  is  not 
to  make  the  mistake  of  regarding  an  action  as  conscious 
merely  because  "  it  looks  so"  or  is  purposeful  and  more  or 
less  highly  coordinated. 

1  The  word  f f  purposeful "  is  used  here  in  the  same  sense  as  in  Chapter  VII 
(p.  70)  and  does  not  include  conscious  purpose  in  its  meaning.  We  shall  see 
that  conscious  purpose  involves  the  cooperation  of  the  cerebrum. 


272  THE  HUMAN  MECHANISM 

The  spinal  cord  alone,  then,  and  without  the  help  of  the 
brain,  is  capable  of  maintaining  a  small  part  of  the  normal 
posture  of  the  resting  frog  and  also  of  executing  some  of 
the  simple  reflexes,  especially  those  involving  movements 
of  the  hind  legs,  but  it  does  not  seem  to  be  capable  of 
originating  actions  or  of  doing  any  except  reflex  actions. 

6.  The  behavior  of  a  frog  with  spinal  cord  and  bulb  only. 
In  this  case  there  is  no  new  feature  in  the  maintenance  of 
posture ;    the  frog  lies  on  its  belly  and  executes  the  same 
reflexes  as  before.   The  respiratory  movements,  however,  go 
on  in  a  normal  manner;  the  vasomotor  tone  of  the  arteries 
is   maintained,    most   vasomotor   reflexes  may  be   produced 
with  ease,  and  the  heart  may  be  reflexly  inhibited.    As  com- 
pared with  the  brainless  frog,  the  number  of  actions  which 
the  animal  can  execute  is  increased,  and  the  reflex  move- 
ments become  somewhat  more  complicated;    but  the  differ- 
ences are  slight  as  compared  with  those  seen  in  the  animal 
which  has  the  'tweenbrain  and  midbrain  in  addition  to  the 
hindbrain  and  cord. 

7.  The  behavior  of  a  frog  with  spinal  cord,  bulb,  midbrain, 
and  'tweenbrain ;  that  is  to  say,  a  frog  with  the  entire  nerv- 
ous system   exclusive   of   the  forebrain,  or  cerebrum.    The 
following  points  are  especially  noteworthy :    (1)  the  sitting 
posture  maintained  at  rest;  (2)  balancing  movements;  and 
(3)  more  complicated  movements  of  locomotion. 

(1)  Such  a  frog,  unlike  those  already  described,  sits  erect 
exactly  like  a  normal  frog;    and  this  fact  shows  that  com- 
plete maintenance  of  the  normal  posture  requires  the  coopera- 
tion of  higher  portions  of  the  nervous  system  than  the  bulb 
and  spinal  cord,  but  does  not  involve  the  cooperation  of  the 
cerebrum. 

(2)  If  the  frog  be  placed  on  a  rectangular  block  of  wood, 
and  the  block  slowly  turned  so  that  the  frog  tends  to  slip 
off  backwards,  it  will  crawl  up   and  over  the   descending 
edge,  keeping  itself  perfectly  balanced.    By  continuing  to 


THE  NERVOUS  SYSTEM  273 

turn  the  block  the  frog  can  be  made  to  creep  around  it 
almost  indefinitely.  Thus  it  not  only  maintains  the  erect 
position  but  also  corrects  loss  of  equilibrium  by  appropriate 
balancing  movements. 

(3)  If  the  frog  be  stroked  upon  its  belly,  it  will  croak; 
if  its  lips  be  touched  with  a  blunt  pin,  it  will  brush  the  pin 
away  with  its  forefoot.  Most  important  of  all,  if  it  be  thrown 
into  the  water,  it  will  swim;  and  when  it  reaches  a  solid 
object  it  will  crawl  out  upon  it  and  come  to  rest.  In  short, 
the  animal  will  carry  out  almost  any  movement  of  which  a 
normal  frog  is  capable,  provided  the  proper  stimulus  is  applied ; 
but  without  this  it  will  do  nothing,  though  capable  of  doing 
so  much. 

The  facts  thus  far  brought  forward  show  that  the  neurones 
of  the  'tween,  mid,  and  hind  brains  and  of  the  spinal  cord 
constitute  nervous  mechanisms  which  can  maintain  the  nor- 
mal posture,  correct  loss  of  balance,  and  even  carry  out  the 
usual  acts  of  locomotion.  The  more  of  the  nervous  system 
which  the  animal  retains,  the  more  complicated  are  the  move- 
ments, as  we  should  expect  when  we  remember  the  increase 
in  the  number  of  neurones  and  the  greater  complexity  of 
coordination  thereby  rendered  possible. 

8.  Comparison  with  the  normal  frog.  The  behavior  of  a 
frog  lacking  only  the  forebrain,  or  cerebrum,  differs  from 
that  of  a  normal  frog  in  two  most  significant  respects.  In 
the  first  place,  the  animal  rarely  makes  any  movement  with- 
out obvious  external  stimulation ;  if  protected  from  drying, 
it  will  often  sit  motionless  for  days,  or  even  weeks.  Such  is 
not  the  conduct  of  an  animal  which  is  aware  of  what  is  going 
on  around  it  or  of  its  own  sensations  or  feelings,  that  is,  of 
a  conscious  animal.  In  the  second  place,  the  frog  shows  the 
most  remarkable  regularity  and  persistency  in  making  re- 
peatedly the  same  response  to  the  same  stimulus ;  if  its  lips 
be  touched  thirty  times  with  a  blunt  needle,  it  will  brush 
at  the  offending  object  every  time  in  the  same  way  with  the 


274  THE  HUMAN  MECHANISM 

same  forefoot.  We  should  certainly  not  expect  a  conscious 
animal  to  do  this ;  for,  after  trying  one  plan  of  action  a  few 
times,  it  would  realize  that  its  efforts  were  unavailing,  and 
would  try  something  else,  such  as  jumping  away.  This  same 
peculiarity  is  met  with  in  all  animals  deprived  of  the  cere- 
brum. They  act  like  mere  complicated  and  faithful  machines ; 
they  do  not  act  as  if  they  were  thoughtful,  original,  or  wise. 

Especially  striking  is  the  avoidance  of  objects  during 
locomotion.  This  fact  looks  at  first  sight  as  if  the  animal 
were  aware  of  the  presence  of  the  obstacle  in  its  path ;  but  a 
dog  without  a  cerebrum,  even  when  it  has  been  without  food 
for  a  day  or  more,  will  go  to  one  side  of  a  piece  of  meat  and 
pass  it  by.  He  acts  as  if  unaware  of  the  nature  of  the  object, 
of  its  use  as  food,  etc.  The  image  of  the  piece  of  meat 
formed  on  his  retina  seems  to  generate  nervous  impulses 
which  pass  to  the  brain  by  way  of  the  optic  nerve  and  re- 
flexly  guide  the  movements  of  the  dog,  but  these  impulses 
do  not  inform  the  animal  of  the  nature  of  the  object,  and 
we  have  no  reason  to  believe  that  the  dog  is  aware  of  the 
existence  of  the  meat. 

When  we  consider  our  own  experience  we  find  that  we 
too,  as  we  walk  along  a  crowded  street,  avoid  objects,  not 
only  without  noticing  them  but  without  even  being  aware 
of  their  presence.  Here  again  the  afferent  impulses  from 
the  retina  pass  to  the  nervous  system  and  reflexly  guide  our 
walking  without  affecting  consciousness  at  all.  And  the 
wonderful  feats  of  somnambulism,  where  the  "  eyes  are 
open "  but  "  their  sense  is  shut,"  where  the  sleeper  main- 
tains his  balance  and  avoids  stumbling  in  situations  where 
he  would  almost  inevitably  fall  if  he  were  aware  of  his 
surroundings,  show  how  perfect  is  this  very  complicated 
mechanism  of  locomotion,  which  seems  to  be  complete  even 
in  the  absence  of  the  cerebrum. 

We  are,  indeed,  so  accustomed  to  regard  our  actions  as 
volitional  and  conscious  that  we  rarely  consider  the  large 


THE  NERVOUS  SYSTEM  275 

part  which  reflexes  from  the  eye,  the  ear,  the  skin,  the 
muscles,  and  the  joints  play  in  guiding  them.  We  will  to 
do  a  certain  thing,  to  walk  to  a  certain  point,  for  example ; 
perhaps  the  first  step  is  a  volitional  act,  but  subsequent 
steps,  the  suiting  of  these  steps  to  slight  unevenness  of  .the 
path,  the  avoidance  of  many  obstacles,  the  maintenance  of 
the  balance  of  the  body  as  a  whole,  —  for  we  walk  not  only 
with  the  legs  but  with  the  entire  body,  —  all  these  things 
take  place  apart  from  any  exercise  of  the  will  and,  for 
the  greater  part,  in  the  entire  absence  of  consciousness, 
although  consciousness  may,  of  course,  at  any  time  inter- 
vene. Reflex  actions  thus  play  a  most  important  part 
even  in  the  execution  of  those  movements  which  we  think 
of  as  distinctly  conscious  acts. 

9.  Connections  of  the  cerebrum  with  lower  portions  of  the 
nervous  system;    "the  way  out."    Granting  that  the  nerv- 
ous events  at   the  basis  of  consciousness  occur  within  the 
cerebrum,  how  do   these   events  influence  the  muscles,  the 
glands,  and  other  organs  which  do  the  bidding  of  the  will? 
What  is  the  way  out  from  this  seat  of  consciousness  ?    This 
path  has  already  been  referred  to  in  Chapter  VII  (p.  81). 
Cells  in  the  gray  matter  of  the   cerebrum  give  off   axons 
which  pass  downward  through  the  structures  of  the  'tween, 
mid,  and  hind  brain  into  the  white  matter  of  the  spinal  cord. 
These  axons  give  off  along  their  course  collaterals  which  end 
in  arborizations  around  nerve  cells  of  the  lower  portions  of 
the  nervous  system  and,  by  bringing  groups  of  these  cells 
into  coordinated  activity,  produce  definite  volitional  move- 
ments.   The  student  should  review  carefully  in  this  connec- 
tion  what   has   already   been   said  with  reference  to   these 
neurones  (see  Fig.  165,  v). 

10.  Connections  of  the  cerebrum  with  lower  portions  of  the 
nervous    system;   "the   way   in."    The    fact    that   afferent 
impulses  from  our  sense  organs  of  sight,  hearing,  etc.  may 
affect    consciousness    indicates    that    there    must    be    some 


276  THE  HUMAN  MECHANISM 

connection  between  afferent  neurones  and  the  cerebral  hemi- 
spheres, since  only  when  the  latter  are  present  does  a  nervous 
impulse  produce  a  conscious  sensation.  The  connection  is  not, 
however,  so  direct  as  in  the  case  of  efferent  impulses.  The 
neurone  of  the  dorsal  root  may  be  traced  as  far  as  the  bulb, 
but  no  farther ;  from  this  point  the  impulse  can  find  its  way 
to  the  cerebrum  only  by  new  neurones,  and  of  these  it  would 
seem  that  there  are  several.  These  relations  are  indicated  in 
Fig.  165,  where  the  efferent  neurones  are  represented  in  black, 
and  the  afferent  in  red. 

This  diagram  brings  out  the  fact  of  increasing  complexity 
of  reflexes  as  we  proceed  to  the  more  anterior  portions  of 
the  nervous  system.  In  the  spinal  cord  the  collaterals  of 
the  afferent  neurone  act  upon  the  efferent  neurones;  in  the 
structures  of  the  midbrain  and  the  'tweenbrain  the  afferent 
tract  makes  connection  with  more  and  more  complicated  and 
extensive  systems  of  these  efferent  neurones  or  motor  mecha- 
nisms. The  range  of  possible  movement  is  increased  to  in- 
clude most  of  the  usual  actions  of  the  animal,  and  some  of 
these  actions  represent  a  very  high  degree  of  coordination. 
Finally,  in  the  cerebrum  the  highest  of  all  these  connections 
is  made;  here  take  place  those  events  of  whose  nature  we 
have  thus  far  been  quite  unable  to  form  any  conception,  but 
which  play  some  part  in  the  genesis  of  conscious  sensations 
and  in  the  closely  related  dispatch  of  volitional  impulses. 
We  can  now  understand  why  it  is  that  removing  this  high- 
est portion  of  the  nervous  system  leaves  untouched  not  only 
the  simpler  reflexes  but  even  the  more  complicated  reflexes 
of  locomotion,  of  swimming,  of  flight,  etc. 

11.  The  nervous  factors  in  locomotion ;  automatic  and 
reflex  elements.  It  is  clear  from  the  considerations  given 
above  that  walking,  running,  and  other  forms  of  locomotion 
are  essentially  nonvolitional  acts,  and  it  is  also  clear  that 
there  must  be  a  nervous  mechanism  capable  of  carrying 
them  out  without  the  aid  of  and  in  the  complete  absence 


THE  NEKVOUS  SYSTEM  277 

of  consciousness.  What  is  the  nature  of  this  mechanism? 
In  answer  to  this  question  we  can  only  make  a  suggestion 
without  pretending  to  give  a  final  explanation. 

In  the  first  place,  walking  obviously  involves  alternate 
steps,  or  forward  thrusts  of  the  body  by  the  two  legs ;  that 
is,  while  one  leg  is  pushing  the  body  forward  by  straighten- 
ing at  hip,  knee,  and  ankle  joints,  the  other  leg  is  bending 
at  these  joints,  the  flexion  of  each  leg  at  the  hip  joint  bring- 
ing it  forward  in  preparation  for  its  next  forward  thrust. 
In  each  leg,  then,  we  have  an  alternation  of  "forward  swing" 
(flexion  at  hip,  knee,  and  ankle)  and  of  "  extensor  thrust " 
(extension  at  hip,  knee,  and  ankle).  In  the  same  leg  the 
flexors  of  the  hip,  knee,  and  ankle  obviously  contract  at 
approximately  the  same  time,  and  the  extensors  at  the  three 
joints  similarly  act  together ;  furthermore,  the  extensor  action 
in  one  leg  is  simultaneous  with  the  flexor  action  in  the 
opposite  leg.  These  actions  may  be  represented  in  diagram 
as  follows: 


\ 

Hip 

Ex. 

Fl. 

Ex. 

Fl. 

4 

i 

43 

Right  leg 

\ 

Knee 
Ankle 

Ex. 
Ex. 

Fl. 
Fl. 

Ex. 
Ex. 

FL 

Fl. 

£H 
PH 
O> 
!H 
O3 

rt 
<o 

g 

d 

•£, 

+j 

[ 

Toes1 

FL 

Ex. 

Fl. 

Ex. 

d 

CJ 

t» 

p 

C3 

CU 

( 

Hip 

Fl. 

Ex. 

Fl. 

Ex. 

'o 
o 

9 
g 

1 

Left  leg 

Knee 
Ankle 

FL 

Fl. 

Ex. 
Ex. 

Fl. 
Fl. 

Ex. 
Ex. 

^ 
« 

t 

"S3 

-1-3 

3 

3 

{ 

Toes 

Ex. 

Fl. 

Ex. 

Fl. 

> 

1 

•J 

Now  it  has  been  shown  that  in  an  animal  made  unconscious 
by  ether  anesthesia,  and  in  which  no  afferent  impulses  may 
enter  the  cord  or  brain,  —  because  of  the  depth  of  anesthesia 
or  even  because  of  cutting  the  dorsal  nerve  roots,  —  similar 
movements  of  the  hind  legs  spontaneously  arise  and  may 

1  Flexion  of  the  toes  in  each  leg  occurs  simultaneously  with  extension  at 
the  other  three  joints,  and  vice  versa.  With  most  people,  owing  to  the  use 
of  improperly  shaped  shoes,  the  toes  are  little  used  in  walking.  See  Chapter 
XXIV  on  the  Hygiene  of  the  Feet. 


278  THE  HUMAN  MECHANISM 

be  kept  up  for  long  periods  of  time.  Evidently  there  is 
a  mechanism  consisting  entirely  of  motor  or  efferent  neurones 
which  by  itself,  independently  of  any  afferent  (that  is,  reflex) 
or  volitional  stimulation,  can  automatically  carry  out  a  large 
part  of  the  act  of  locomotion.  Locomotion  becomes  funda- 
mentally the  act  of  an  automatic  mechanism  and  is  com- 
parable to  the  alternate  automatic  contractions  of  inspiration 
and  expiration. 

The  parallel  between  the  automatism  of  respiration  and 
the  automatism  of  locomotion  becomes  still  more  striking 
when  we  find  in  both  cases  that  afferent  impulses  do  actually 
intervene  to  guide  and  so  make  more  exact  and  efficient 
the  fundamental  automatic  movements.  Thus  we  know  that 
afferent  impulses  started  by  the  expansion  of  the  lungs 
during  inspiration  check  the  inspiratory  effort  then  in  prog- 
ress and  so  bring  on  the  next  expiration  sooner  than  it  would 
automatically  occur.  Similarly,  the  pressure  upon  the  sole  of 
the  foot  as  it  touches  the  ground  reflexly  guides  and  proba- 
bly strengthens  the  automatic  extensor  thrust;  and  many 
other  reflexes  through  the  cord  are  known  to  serve  similar 
functions. 

To  sum  up,  then:  The  action  of  the  legs  in  locomotion 
seems  to  be  fundamentally  an  automatic  action,  but  these 
automatic  movements  are  guided  by  afferent  impulses  which 
stream  in  from  skin,  muscles,  and  joints  as  the  act  pro- 
gresses. Just  as  we  can  volitionally  hold  the  breath,  so  we 
can  volitionally  start  or  stop  walking;  or  just  as  we  can 
volitionally  change  the  depth  and  rhythm  of  respiration, 
so  we  can  volitionally  change  the  pace  or  length  of  stride; 
or  just  as  a  dash  of  cold  water  on  the  skin  or  the  presence 
of  an  irrespirable  gas  reflexly  changes  the  character  of  the 
breathing  movements,  so  unevenness  of  the  path  or  visual 
impulses  from  an  object  in  the  way  changes  the  character  of 
the  locomotion.  In  all  cases  reflex  and  volitional  interference 
acts  on  a  fundamental  automatic  nervous  mechanism. 


THE  NERVOUS  SYSTEM  279 

12.  The  maintenance  of  balance  and  the  regulation  of  mus- 
cular tone.    Walking,  however,  involves  more  than  the  action 
of  the  neuromuscular  mechanisms  of  the  legs ;    for  here,  as 
weU  as  in  complicated  volitional  actions,  the  balance  of  the 
body  must  be  preserved.    For  this  reason  we  swing  the  arms 
and  execute  ever-changing  contractions  of  the  muscles  of  the 
trunk.    Moreover,  a  proper  state  of  tonic  contraction  in  each 
muscle  is  necessary  to  the  proper  execution  not  only  of  the 
act  of  walking  but  of  other  acts  as  well,  whether  these  are 
volitional  or  nonvolitional.    Into  the  mechanism  of  this  won- 
derfully perfect  function  of  the  body  we  cannot  go  within 
the  limits  of  the  present  book;    but  there  is  good  ground 
for  thinking  that,  at  least  in  the  mammals,  the  cerebellum 
is   a  very  important  and  probably  the   all-important  organ 
concerned  in  effecting  these  coordinations. 

13.  Actions   resulting    from    nervous   processes   originating 
within  the  cerebrum.    A  very  large  part  of  the  activities  of 
the  body  are  thus  fundamentally  reflex  actions ;  they  do  not 
require  the  aid  of  consciousness  for  then*  execution.    And  it 
is  fortunate  for  us  that  this  is  the  case;   one  has  only  to 
imagine  a  human  being  who  has  to  give  his  attention,  or 
"his  mind,"  as  we   often  say,  to   every  adjustment  of  the 
digestive,  respiratory,  and  vascular  systems  required  to  meet 
the  changing  necessities  of  life ;  who  has  to  keep  his  thoughts 
on  every  movement  of  walking  or  running;  who  has  to  be 
constantly  on  guard  against  loss  of  balance  even  when  sitting 
still.    Such  a  being  is  almost  inconceivable ;    he  would  "  go 
crazy "  in  a  single  day ;    but  we  can  in  this  way  realize  to 
what  extent  the  reflex  mechanisms  of  the  body  perform  the 
menial  offices  of  life,  leaving  the  mind  free  for  higher  things. 

Speech  is  the  result  of  movements  in  which  the  muscles 
of  respiration,  those  of  the  larynx,  those  of  the  tongue, 
and  those  of  the  lips  cooperate  to  produce  articulate  and 
intelligible  sound.  The  act  of  writing  also  consists  of  a  series 
of  movements  in  which  the  muscles  of  the  arm  and  hand 


280  THE  HUMAN  MECHANISM 

cooperate  to  make  thought  visible;  performing  on  a  musical 
instrument,  modeling  a  figure  in  clay  or  marble  or  bronze, 
painting  a  picture  —  all  these  things  occur  to  us  as  examples 
of  movements  which  are  fundamentally  neither  reflex  nor 
automatic.  Such  are  the  highest  actions  of  the  body,  and 
the  movements  of  which  these  actions  are  made  up  are 
chosen  and  directed  by  the  will. 

These  higher  actions,  like  consciousness,  depend  upon  the 
presence  of  the  forebrain.  When  a  certain  area  of  the  cere- 
brum is  destroyed  by  disease,  the  power  of  speech  is  lost; 
when  another  part  is  destroyed,  the  skilled  use  of  the  hand 
is  lost ;  destruction  of  other  portions  affects  in  the  same  way 
others  of  these  skilled  movements.  In  such  cases  locomotion, 
the  maintenance  of  balance,  the  movements  of  respiration, 
etc.  may  be  and  usually  are  unaffected;  the  patient  merely 
loses  the  power  of  doing  one  or  more  of  those  things  which 
involve  the  selection  of  disconnected  and  to  some  extent 
independent  movements  giving  expression  to  some  original 
thought,  sentiment,  or  idea. 

The  neurones  of  the  cerebrum  and  their  connections  thus 
constitute  nervous  mechanisms  whose  activity  is  essential  to 
consciousness, — to  our  seeing,  our  hearing,  our  smelling,  and, 
more  than  this,  to  our  understanding  of  what  we  see,  or  hear, 
or  smell,  —  nervous  mechanisms  whose  activity  is  also  neces- 
sary to  the  expression  of  our  thought  in  action.  It  is  because 
of  this  fact  that,  when  the  cerebrum  is  removed,  the  animal 
becomes  merely  a  complicated  reflex  machine,  acting  only 
as  it  is  immediately  stimulated  from  without  or  by  events 
taking  place  within  its  own  body. 

14.  Effects  of  anesthetics  on  the  nervous  system.  When 
a  person  passes  under  the  influence  of  an  anesthetic,  the 
first  function  to  disappear  is  consciousness;  the  ether  or  the 
chloroform  first  paralyzes  this  highest  and  most  complex 
connection  between  the  afferent  and  the  efferent  sides  of 
the  nervous  system.  In  this  condition  the  patient  may  groan 


THE  NERVOUS  SYSTEM  281 

and  struggle,  for  he  is  in  somewhat  the  same  state  as  the 
animal  without  cerebral  hemispheres.  The  use  of  the  sur- 
geon's knife  will  still  produce  movements;  respiration  may 
be  affected  so  as  to  result  in  groans  and  other  movements 
which  the  inexpert  observer,  perhaps  in  alarm,  attributes  to 
severe  suffering ;  and  yet  when  the  patient  awakes  he  tells 
us  he  knew  nothing  of  what  passed  and  felt  no  pain.  It  is 
important  to  realize  that  the  signs  of  pain  are  never  reliable 
evidence  of  its  existence. 

If  the  anesthesia  be  pushed  further,  even  these  more 
complicated  reflexes  disappear.  In  the  ordinary  major  opera- 
tions of  surgery  the  ether  or  the  chloroform  is  given  until 
it  interrupts  not  only  the  cerebral  connections  between  the 
afferent  and  efferent  paths  but  also  those  of  the  lower  por- 
tions of  the  brain;  it  is  even  administered  until  only  a  few 
reflexes  are  left,  such  as  the  wink  when  the  cornea  is  touched, 
the  contraction  of  the  pupil  when  the  eye  is  exposed  to  light, 
etc.  —  these  serving  as  useful  tests  of  the  condition  of  the 
patient.  If,  for  example,  the  pupil  no  longer  contracts  to 
light,  it  is  an  indication  that  the  anesthesia  is  going  too  far 
—  too  near  the  point  where  the  nervous  mechanism  of  respi- 
ration, etc.,  will  be  paralyzed.  The  giving  of  ether  is  then 
suspended  until  these  reflexes  are  again  well  established. 

After  the  operation,  as  the  ether  or  chloroform  is  elimi- 
nated from  the  system,  the  reflexes  return  in  the  reverse 
order;  and  the  unconscious  movements,  groans,  incoherent, 
or  even  more  or  less  coherent,  talking  (comparable  with 
talking  in  one's  sleep)  are  sometimes  most  harrowing  to  the 
feelings  of  those  who  do  not  understand  that  they  are  all 
unconscious  acts.  The  physician  and  nurse  who  remain  un- 
moved may  even  be  wrongly  charged  with  lack  of  feeling 
because  they  do  not  waste  sympathy  where  they  know  there 
is  neither  suffering  nor  consciousness. 

15.  Inhibitory  phenomena  in  the  nervous  system.  We  have 
learned  that  some  nerves  excite  organs  to  activity,  while 


282  THE  HUMAN  MECHANISM 

others  diminish  activity  or  abolish  it  altogether  (p.  160). 
The  beat  of  the  heart  is  quickened  by  one  set  of  nerves 
and  slowed  by  another;  the  circular  muscular  fibers  of  the 
arterioles  are  excited  to  contract  by  vasomotor  nerves,  their 
tonic  constriction  is  paralyzed  or  inhibited  by  vasodilators, 
and  many  other  examples  might  be  drawn  from  the  action 
of  neurones  on  peripheral  organs  of  the  body. 

Precisely  the  same  thing  is  true  in  the  brain  and  spinal 
cord.  Afferent  impulses  may  not  only  reflexly  excite  neu-, 
rones  to  activity  but  may  also  inhibit  the  existing  or  threat- 
ened activity  of  other  neurones,  as  when  a  sneeze  is  stopped 
by  biting  the  upper  lip  or  by  pinching  the  nose;  or  an 
action  may  be  inhibited  by  a  volitional  impulse  from  the 
cerebrum,  as  when  the  breathing  movements  are  voluntarily 
stopped  for  a  while,  or  when  we  similarly  stop  a  wink  or 
a  sneeze.  These  are  all  examples  of  inhibition,  not  of  the 
skeletal  muscles  concerned  but  of  the  neurones  which  inner- 
vate them  —  in  other  words,  of  the  inhibition  of  one  neurone 
by  another. 

It  must  be  understood  that  inhibition  is  as  essential  a 
part  of  the  activity  of  the  nervous  system  as  is  excitation. 
Just  as  the  driver  of  a  team  must  urge  on  one  horse  while 
he  restrains  another,  so  in  all  more  complicated  actions, 
probably  in  all  actions,  reflex  or  volitional,  the  orderly 
movement  is  as  much  the  result  of  holding  one  neurone  in 
check  as  of  stimulating  another  one  to  work,  or  to  work 
harder.  Consciousness  proves  its  presence  most  conclusively 
by  suppressing  reflexes  which  would  otherwise  inevitably 
occur  and  by  bringing  about  new  movements  to  meet  the 
desired  end.  Even  in  the  highest  processes  of  the  most 
highly  organized  of  nervous  systems,  namely,  those  in  which 
human  action  originates,  the  man  reveals  his  character  and 
influences  the  world  around  him  by  what  he  does  not  do  — 
by  what  he  refrains  from  doing,  sometimes  at  the  cost  of 
severe  struggle  against  impulse,  instinct,  or  passion  —  quite 


THE  NERVOUS  SYSTEM  283 

as  much  as  by  what  he  does.  Education,  even,  has  been 
partially  denned  as  the  "training  of  inhibitions  and  the 
control  of  reflexes." 

16.  The  cerebrum  the  chief  organ  for  the  acquisition  of  new 
coordinations  and  associations.    It  would,  however,  be  taking 
too  narrow  a  view  of  the  functions  of  the  cerebrum  to  regard 
it    simply   as   the    seat   of    consciousness   and    volition.    In 
Chapter  VII,  §  15,  we  saw  that  hi  addition  to  definite  in- 
herited reflex  mechanisms,  such  as  those  of  winking,  —  the 
so-called  unconditioned  reflexes,  —  new  paths  of  conduction 
from  the  afferent  to  the  efferent  side  are  acquired   during 
life  by  the  repeated  association  of  two  acts.    Doubtless  all 
parts  of  the  brain  and  spinal  cord  possess  in  some  degree 
this  power  of  making  new  associations  like  those  concerned 
in  the  conditioned  reflexes ;   but  the  cerebrum  is  certainly 
the  organ  in  which  they  are  made  most  readily,  and  there 
can  be  no  doubt  that  one  of  its  chief  functions  is  the  acqui- 
sition of  such  new  paths  of  conduction  as  the   experience 
and  activities  of  life  first  blaze  within  its  nervous  substance 
and  subsequently,  by  the  repeated  passage  of  nervous  im- 
pulses over  the  "  blazed  trail,"  change  to  "  beaten  paths  "  of 
easy  conduction.    Here  every  act  and  experience  of  life  may 
leave  its  record,  and  here  good  and  bad  habits  are  acquired. 

17.  Use  and  disuse  as  factors  in  individual  development, 
training,  and  efficiency.    When  we  consider  the  marvelously 
complicated    character   of    the    nervous    mechanisms    which 
control  our  actions,  we  naturally  wonder  how  this  intricate 
machinery  can  be  built  and  why  it  does  not  more  frequently 
get  out  of  order.    We  cannot  say  that  a  simple  and  compre- 
hensive answer  will  not  some  day  be  given  to  these  ques- 
tions,  but   to-day  we  have  no  adequate   answer  whatever. 
The  neurones  with  which  we  must  work  in  life  are  born  with 
us ;  but  in  most  cases  efficient  connections  must  subsequently 
be  made  between  them,  thus  perfecting  the  mechanisms  they 
compose ;  and  this  perfecting  of  the  nervous  machine  comes 


284  THE  HUMAN  MECHANISM 

with  use.  The  use  of  a  nervous  mechanism  is  generally 
essential  to  its  proper  development,  just  as  the  use  of  a 
muscle  is  essential  to  its  strength.  If  the  child  never  tried  to 
walk,  the  neurones  which  carry  out  the  movements  of  walk- 
ing would  not  develop ;  not  only  do  the  muscles  of  an  arm 
strapped  down  to  the  side  of  the  body  waste  away  and  be- 
come practically  bands  of  connective  tissue,  but  the  neurones 
concerned  in  the  actions  which  the  arms  should  execute 
degenerate  and  may  ultimately  be  irreparably  injured. 

Provision  is  made  from  earliest  life  for  the  proper  develop- 
ment of  these  neurones  and  the  establishment  of  irritable 
connections  between  them  by  use;  out  of  the  first  aimless 
movements  of  the  head  and  eyes  and  hands  and  legs  of  the 
baby  the  simpler  coordinating  nervous  mechanisms  are  one 
by  one  brought  to  perfection ;  then  comes  the  training  of 
those  reflexes  which  maintain  the  erect  position  and  of  those 
nervous  mechanisms  which  govern  locomotion;  then  play 
comes  in,  with  its  ceaseless  activity,  increasing  still  further 
the  number  of  movements  which  the  nervous  system  can 
make  and  correspondingly  enlarging  the  possibility  of  human 
achievement.  As  the  child  grows  older  the  family  calls 
upon  him  to  contribute  some  share  to  its  life  or  support; 
new  activities,  in  the  shape  of  chores  about  the  house  or  the 
farm,  now  share  with  play  the  work  of  the  nervous  system ; 
activity  becomes  less  general,  more  special.  Finally  the 
youth  settles  down  to  some  definite  occupation  or  pursuit, 
and  the  more  strictly  this  is  adhered  to,  the  narrower  be- 
comes the  range  of  activity;  the  more  constantly  a  few  sys- 
tems of  neurones  are  used,  the  more  rarely  are  others  called 
into  play. 

18.  The  physical  basis  of  habits.  All  this  indelibly  writes 
its  history  in  the  nervous  system.  No  fact  is  more  significant 
or  of  greater  physical  and  moral  import  than  that  the  doing 
of  any  act  so  affects  the  connections  of  neurones  with  one 
another  as  to  make  it  easier  to  do  the  same  act  again  under 


THE  NERVOUS  SYSTEM  285 

the  same  conditions ;  that  refraining  from  doing  something 
toward  which  we  are  inclined  similarly  renders  more  easy 
the  inhibitory  processes  concerned  when  the  same  conditions 
impel  us  toward  it  again.  We  are  largely  what  we  make 
ourselves  by  the  training  which  our  actions  give  to  the 
nervous  system. 

And  what  activity  thus  does  for  the  development  of  power 
it  does  also  for  the  maintenance  of  power.  An  efficient  nerv- 
ous mechanism  of  any  kind  once  acquired  does  not  remain 
efficient  without  use.  The  man  who  has  developed  a  rugged 
constitution  in  colder  climates  and  then  lives  for  years  in 
the  tropics,  constantly  exposed  to  a  warm  climate,  finds  on 
return  to  the  home  of  his  youth  that  the  mechanism  of  heat 
regulation  does  not  readily  adjust  itself  to  cold  damp  winds 
and  blizzards;  the  athlete  who  has  learned  to  execute  the 
greatest  variety  of  "  tricks "  in  the  gymnasium  and  then 
settles  down  to  a  sedentary  life  finds  after  some  years  that 
he  is  almost  as  helpless  as  the  man  who  gave  no  attention 
to  such  training.  It  is  unnecessary  to  multiply  examples. 
Efficiency  in  any  direction  is  the  result  of  continued  use  of 
organs  and  especially  of  continued  training  of  the  nervous 
system.  As  we  fit  ourselves  to  do  some  few  things,  and  to 
do  them  well,  we  have  not  time  to  conserve  by  use  the  effi- 
ciency of  all  the  nervous  mechanisms  we  have  acquired ;  we 
must  to  some  extent  sacrifice  the  more  general  actions  for 
those  which  are  more  special  and  useful.  But  it  must  not 
be  forgotten  that  this  can  be  carried  too  far ;  that  a  certain 
amount  of  general  activity  is  a  condition  of  healthy  living  and 
that  one  of  the  problems  of  life  to  solve,  and  to  solve  aright, 
is  how  to  distribute  our  activity  between  the  two.  To  the 
consideration  of  these  questions  we  shall  return  in  our  study 
of  personal  hygiene. 


CHAPTER  XVI 
FOOD  ACCESSORIES,  DKUGS,  ALCOHOL,  AND  TOBACCO 

1.  Food  accessories  and  drugs.  Through  the  alimentary 
and  respiratory  tracts  there  are  received  into  the  blood  not 
only  substances  such  as  proteins,  gelatin,  fats,  carbohydrates, 
salts,  and  water,  which  we  have  described  as  supplying  the 
material  for  power  and  for  growth  and  repair,  but  also  other 
substances  capable  of  modifying  in  one  way  or  another  the 
course  of  events  within  the  body.  The  flavors  which  con- 
tribute to  the  enjoyment  of  foods  play  an  important  r61e  in 
the  secretion  of  the  gastric  juice,  and  yet  the  substances 
which  cause  these  flavors  are  negligible  as  sources  of  power. 
Salt  belongs  under  the  same  head,  for  we  use  in  cooking 
more  salt  than  is  needed  to  make  good  the  daily  loss  from 
the  body,  and  we  do  this  to  develop  an  agreeable  flavor  in 
our  food.  Substances  of  this  kind  are  spoken  of  as  food 
accessories,  and  among  them  must  be  included  coffee  and  tea, 
for  their  effect  is  not  chiefly  a  matter  of  nutrition ;  certain 
constituents  of  tea  and  coffee  absorbed  into  the  blood  affect 
the  nervous  system,  and  it  is  largely  for  this  reason  that  we 
use  them. 

We  may  pass  in  this  way  from  the  necessary  food  acces- 
sories through  those,  like  coffee  and  tea,  which,  while  not 
essential,  may  still  be  regarded  as  part  of  the  food  of  a  large 
portion  of  mankind,  to  the  great  number  of  chemical  com- 
pounds known  as  drugs,  which  also  act  by  changing  the 
course  of  events  within  the  body ;  and  it  is  difficult  to  draw 
any  sharp  line  of  distinction  between  those  which  occasionally 
serve  as  medicine  or  "  stimulants  "  and  those  of  which  daily 
use  is  made  as  food  accessories. 

286 


FOOD  ACCESSORIES  AND  DRUGS  287 

Animals  as  a  rule  take  substances  into  their  bodies  only 
to  satisfy  hunger  or  thirst  or  appetite ;  man  alone  takes,  in 
addition  to  his  nutriment,  food  accessories  and  drugs  for 
the  sake  of  their  special  effect  upon  the  nervous  system  or 
other  organs.  Many  of  the  numerous  food  accessories  which 
human  ingenuity  has  discovered  or  devised  are  harmless 
enough  in  the  form  used,  but  others  contain  substances 
which  are  capable  of  poisoning  the  body.  It  is  an  important 
part  of  the  study  of  personal  hygiene  to  learn  of  what 
these  substances  consist,  what  is  their  action  on  the  human 
organism,  and  wherein  lie  their  special  dangers. 

2.  The   drug  habit.    It   is  a   lamentable   fact   that   large 
amounts  of  drugs  are  swallowed  by  men  and  women  apart 
from  any  medical  need  which  compels  their  use.    In  a  sub- 
sequent chapter  we  shall  show  reasons  for  avoiding  an  undue 
dependence  upon  drugs  as  a  remedy  for  various  minor  ills. 
Bad  as  this  practice  is,  with  its  tendency  to  rely  upon  the 
uncertain  action  of  a  drug  instead  of  taking  proper  hygienic 
care  of  the  body,  it  is  far  worse  to  make  habitual  use  of 
drugs  for  their  special  effects  upon  the  healthy  body,  for 
the  habit  is  one  which  is  only  too  easily  cultivated.    There 
is  no  reason  why  a  healthy  human  being,  living  a  normal 
lif e  amid  healthful  surroundings,  should  need  to  use  drugs 
habitually,    and   a   little    consideration   will   show   that   the 
practice  is  dangerous. 

3.  Dangers  of  the  drug  habit.   When  we  eat  meat  or  vege- 
tables,   or   when   we   breathe   air,    we    take   into   the   body 
materials    needed    for    normal    living.     These    things    have 
always   formed  part   of   the   food   of  the   race   and,   unless 
wrongly  taken,  do  good  and  not  harm.    When,  on  the  other 
hand,  we  take  a  drug,  such  as  chloroform,  or  cocaine,  or 
opium,  or  alcohol,  or  coffee,  or  tea,  we  take  something  which 
is  foreign  to  the  body,  in  so  far  as  it  has  not  been  a  regular 
constituent  of  animal  food  in  the  past.    It  is  not  needed,  as 
protein  and  salt  and  water  are  needed;  there  is  no  special 


288  THE  HUMAN  MECHANISM 

preparation  for  its  reception ;  and  while  it  may  do  good, 
there  is  danger  that  it  may  do  harm. 

In  the  second  place,  the  exact  action  of  many  drugs  is 
only  imperfectly  understood.  In  an  emergency  the  physi- 
cian uses  them  temporarily,  for  some  effect  which  he  desires 
to  produce,  thus  tiding  over  a  difficulty.  He  uses  the  drug 
only  a  few  times,  at  most,  and  is  consequently  not  greatly 
concerned  about  unfavorable  attendant  effects ;  it  accom- 
plishes some  needed  purpose,  and  if  it  does  any  harm,  the 
organism  may  be  trusted  to  recover  from  it.  It  is  very  dif- 
ferent, however,  with  the  habitual  use  of  any  drug.  The  very 
fact  that  it  gives  some  new  direction  to  the  events  taking 
place  within  the  body  means  that  abnormal  conditions  of  life 
are  being  maintained,  and  we  have  already  learned  that 
abnormal  conditions  of  life  are  apt  to  be  unhygienic. 

Again,  the  use  of  drugs  is  only  too  apt  to  be  substituted 
for  the  hygienic  conduct  of  life.  We  may,  for  example, 
take  drugs  to  accomplish  something  which  the  healthy  body 
should  accomplish  for  itself  without  outside  help.  When 
one  drinks  a  cup  of  black  coffee  to  facilitate  mental  work 
which  his  fatigued  condition  would  not  otherwise  allow 
him  to  do,  he  is  trying  to  get  from  a  drug  the  power  which 
he  could  and  probably  should  secure  by  normal  sleep.  The 
coffee  acts  like  a  whip  to  a  tired  horse ;  the  same  work 
is  done  as  might  have  been  done  had  the  horse  been  allowed 
a  little  rest,  but  the  horse  is  not  as  well  off  when  he  does 
the  work  under  the  lash  as  when  he  does  it  in  a  properly 
rested  condition.  Similarly,  persons  suffering  from  sleepless- 
ness often  take  drugs  used  to  produce  sleep  (hypnotics), 
and,  superficially  at  least,  the  sleep  thus  secured  resembles 
normal  sleep;  but  experience  shows  that  few  if  any  hypnotics 
can  be  used  for  any  length  of  time  without  bad  effects. 
Here  again  a  drug  is  being  depended  upon  to  do  what  the 
normal  body  should  do  for  itself.  Pepsin  tablets  may  be 
taken  to  aid  digestion,  and  thereby  an  attack  of  indigestion 


FOOD  ACCESSORIES  AND  DRUGS  289 

may  sometimes  be  prevented  or  relieved;  but  a  healthy 
stomach  should  furnish  its  own  pepsin,  and  the  fact  that  it 
does  not  do  so  is  a  sure  warning  that  something  is  wrong 
in  the  conduct  of  life.  It  is  irrational  to  neglect  the  duty 
of  attending  to  the  cause  of  the  ailment,  and  it  is  foolish  to 
substitute  temporary  relief  for  permanent  cure.  Perhaps  if 
the  drug  did  all  that  the  proper  care  of  the  body  does,  and 
did  no  more,  no  serious  objection  could  be  made  to  its  use ; 
but  there  is  probably  no  drug  of  w^hich  this  is  true,  and  for 
this  reason  it  is  foolish  and  rash  to  try  to  substitute  the  use 
of  drugs  for  the  hygienic  conduct  of  life. 

Lastly,  if  the  drugs  do  not  accomplish  in  the  long  run 
what  should  be  done  by  the  hygienic  conduct  of  life,  their 
extensive  use  becomes  all  the  more  dangerous  in  view  of 
the  unquestioned  fact  that  we  are  apt  thereby  to  become 
their  slaves.  Every  man  is  the  slave,  broadly  speaking,  of 
the  habits  he  forms,  and  it  is  only  a  question  as  to  whether 
he  will  be  the  willing  slave  of  good  habits  or  the  abject 
slave  of  bad  habits.  The  man  who  leads  a  hygienic  life  is 
the  slave  of  muscular  activity,  of  correct  feeding,  of  proper 
clothing,  of  rest,  etc. ;  that  is  to  say,  these  things  become 
necessary  to  his  life ;  he  cannot  get  along  without  them. 
If  for  these  proper  agents  of  health  he  persistently  sub- 
stitutes some  drug,  whether  it  be  alcohol,  or  tobacco,  or 
coffee,  or  tea,  or  chocolate,  or  opium,  the  habit  of  using 
the  drug  is  substituted  for  that  of  maintaining  normal 
conditions.  But  since  drugs  cannot  entirely  take  the  place 
of  such  conditions,  the  constitution  goes  from  bad  to  worse, 
and  increasing  dependence  must  be  placed  upon  the  drug. 
It  is  a  safe  rule  that  whenever  we  are  uncomfortable  or 
unhappy  without  the  use  of  a  certain  drug  we  should  cease 
using  it  until,  with  the  help  of  hygienic  living,  we  can  get 
along  without  it. 

There  are  people  who  are  slaves  of  coffee,  of  tea,  of 
chocolate,  of  patent  medicines,  of  candy,  and  of  soda  water 


290  THE  HUMAN  MECHANISM 

just  as  truly  as  there  are  slaves  of  tobacco,  or  of  alcohol, 
or  of  opium.  It  is  worse  to  be  the  slave  of  alcohol  than  of 
coffee,  because  the  evil  consequences  of  alcohol  are  greater 
than  those  produced  by  the  corresponding  use  of  coffee; 
but  it  is  by  the  same  process  in  both  cases  that  the  man 
or  woman  becomes  a  slave  to  the  drug,  and  that  process  is 
the  formation  of  bad  habits. 

With  these  practical  considerations  about  the  use  of  drugs 
—  by  which  term  it  will  be  seen  that  we  mean  not  simply 
the  medicines  purchased  from  the  apothecary  but  all  those 
substances  which  are  taken  into  the  body  in  order  to  give 
some  new  or  abnormal  direction  to  the  course  of  events  in 
the  organism  —  we  may  pass  on  to  the  discussion  of  those 
in  common  use. 

4.  Tea  and  coffee.  Different  as  are  these  drinks  in  taste 
and  appearance,  their  most  important  physiological  effects 
are  due  essentially  to  the  same  substances ;  namely,  caffeine 
(or  theine)  and  tannic  acid  (or  tannin).  Caffeine  is  a  very 
powerful  stimulant,  especially  of  the  nervous  system  and 
also  of  the  heart,  although  probably  to  a  lesser  degree ; 
tannin,  on  the  other  hand,  is  a  bitter,  astringent  substance, 
which  may  considerably  hinder  digestion  and  directly  injure 
the  mucous  membrane  of  the  stomach.  Tea  contains  about 
twice  as  much  tannin  as  an  equal  weight  of  coffee,  but  as 
coffee  is  frequently  made  much  stronger  than  tea,  the  actual 
amount  per  cup  may  often  be  more  nearly  equal  in  the  two 
drinks  than  these  figures  indicate.  The  amount  of  tannin 
dissolved  in  tea  varies  greatly  with  the  method  of  prepara- 
tion, and  largely  for  this  reason  tea  should  not  be  boiled 
nor  allowed  to  steep  too  long.  The  proper  method  of  making 
tea  is  to  pour  over  the  dry  leaves  water  which  has  been 
brought  just  to  the  boiling  point  and  then  to  allow  the 
infusion  to  stand,  without  further  heating,  for  not  more 
than  a  few  minutes. 

Both  tea  and   coffee   seem  to  have   a  slightly  retarding 


FOOD  ACCESSORIES  AND  DRUGS  291 

influence  upon  gastric  digestion.  In  healthy  people  this 
is  of  little  consequence,  but  when  the  digestive  powers  are 
in  any  way  impaired,  the  use  of  these  beverages  may  be 
inadvisable.  The  more  important  effect,  however,  of  both 
tea  and  coffee  is  in  their  stimulating  action  on  the  nervous 
system.  No  satisfactory  explanation  has  yet  been  given  of 
the  fact  that  some  people  can  use  tea  and  not  coffee,  while 
with  others  the  reverse  is  true.  It  is  probably  safe  to  say 
that  when  used  in  moderation,  tea  and  coffee  are  usually 
harmless  to  those  leading  an  otherwise  hygienic  life.  They 
should  be  used  sparingly  by  nervous  people  and  by  those  in 
whom  digestion  is  feeble  and  slow  (Hutchinson).  Even  by 
the  perfectly  healthy  they  should  not  be  used  to  excess,  nor 
should  the  habit  be  acquired  of  using  them  as  the  whip  to 
the  tired  horse.  Drinking  strong  coffee  in  order  to  keep  awake 
for  evening  study  is  objectionable,  and  the  substitution  of 
afternoon  tea  for  a  little  rest  or  sleep  is  also  unwise. 

5.  Cocoa  is  made  from  the   seeds  of  trees  of  the  genus 
Theobroma,   and   chocolate   is   prepared .  from   cocoa.    In   the 
solid    form   both   are   highly   nutritious,    as   shown   by    the 
following  average  results  of  analyses: 

PROTEIN  FAT       CARBOHYDRATE 

Cocoa 21.6%  28.9%  37.7% 

Chocolate 12.9%  48.7%  30.3% 

When  used  as  a  beverage,  however,  the  nutriment  derived 
from  them  is  small.  In  addition,  cocoa  and  chocolate  both 
contain  theobromine,  a  substance  closely  related  chemically  to 
caffeine  and  possessing  much  the  same  stimulating  proper- 
ties. In  general,  the  same  hygienic  considerations  which 
apply  to  the  use  of  tea  and  coffee  should  guide  us  also  in 
the  use  of  chocolate  and  cocoa. 

6.  Soda  water  and  similar  beverages.    Of  these  little  need 
be  said.    In  general,  they  are  harmless  enough,  especially  to 
those  enjoying  perfect  digestion.    The  large  amount  of  sugar 


292   '  THE  HUMAN  MECHANISM 

which  they  contain  is  apt  to  make  matters  worse  in  many 
cases  of  dyspepsia;  by  taking  them  frequently  between 
meals  the  appetite  for  wholesome  food  is  impaired,  and 
excessive  indulgence  in  them  under  any  circumstances  is 
needless  and  foolish. 

7.  Alcoholic  beverages.    In  the  case  of  an  alcoholic  drink 
we  have  to  deal  with  something  which,  like  tea  and  coffee 
and  cocoa  and  "  temperance  drinks,"  is  used  as  a  beverage, 
and   to   that   extent   must   be   classed  in   the   same   group. 
Alcoholic  drinks  are,  however,  taken  as   stimulants  and  so 
resemble  tea  and  coffee  and  cocoa,  but  they  differ  from  all 
of   these   in   their  action  upon   the   body.    Moreover,   their 
abuse   gives  rise   not   only   to   degraded   moral   and   social 
conditions,  but  is  also  attended  with  bad  hygienic  effects. 
Everyone   should  be   informed  of  their  nature   and  of  the 
dangers  attending  their  use. 

The  common  alcoholic  beverages  consist  of  (1)  malt 
liquors,  including  beer  and  ale;  (2)  wines,  such  as  hock, 
claret,  Burgundy,  sherry,  and  champagne ;  (3)  distilled 
liquors,  including  brandy,  whisky,  rum,  and  gin ;  and 
(4)  liqueurs  and  cordials.  These  groups  are  distinguished 
from  one  another  largely  by  the  method  of  preparation  and 
by  the  amount  of  alcohol  they  contain.  Malt  liquors  are 
fermented  liquors  which  contain  from  three  to  eight  per  cent 
of  alcohol;  wines  are  also  fermented  liquors,  but  contain 
from  seven  to  twenty  per  cent  of  alcohol;  distilled  liquors, 
on  the  other  hand,  are  first  fermented  and  then  concen- 
trated by  distillation,  and  contain  from  thirty  to  sixty-five 
per  cent  of  alcohol.  In  all  these  the  most  important  con- 
stituent, so  far  as  their  physiological  action  upon  the  body 
is  concerned,  is  the  chemical  compound  known  as  ethyl 
alcohol  (C2H6O  or  C2H6 .  OH). 

8.  Fermentation.    The  ethyl  alcohol  in  each  of  these  bev- 
erages  is  produced  by  the   action   of   yeast  on   sugar,  and 
this  action  is  known  as  alcoholic  fermentation.    Yeast  is  a 


FOOD  ACCESSORIES  AND  DRUGS  293 

unicellular  plant,  and  when  a  small  amount  of  it  is  added 
to  a  solution  of  grape  sugar  or  fruit  sugar,  it  breaks  up  these 
substances,  chiefly  into  alcohol  and  carbon  dioxide  gas.  The 
latter  passes  off,  while  the  alcohol  remains  behind  in  the 
solution.  In  addition  to  these  chief  products  of  fermenta- 
tion there  are  always  formed  other  products  in  small  quan- 
tities, and  to  these,  in  part,  the  flavor  of  the  fermented 
mixture  is  due.  Different  varieties  of  yeast  produce  dif- 
ferent kinds  of  fermentation.  Thus  one  variety  (domesti- 
cated yeast)  is  used  in  making  beer,  and  another  (wild 
yeast)  in  making  wine.  The  amount  of  alcohol  produced 
differs  with  the  yeast  used,  as  do 
also  the  character  and  quantity  of 
the  secondary  products.  The  growth 
of  yeast,  like  that  of  all  living  fer- 
ments, is  checked  by  the  accumu- 
lation of  the  products  of  its  own 
activity.  Consequently  when  the  al- 
cohol produced  reaches  a  certain  per- 
centage (usually  less  than  ten  per  FIG.  117.  Yeast  cells 
cent)  the  fermentation  ceases.  Alco- 
holic drinks  which  contain  higher  percentages  of  alcohol  are 
prepared  by  special  processes,  which  will  be  described  later. 
9.  Malt  liquors.  Malt  consists  of  sprouted  grains  (chiefly 
barley).  The  grains  contain  a  large  amount  of  starch  which 
during  the  process  of  germination  is  converted  into  sugar 
by  diastase,  an  enzyme  produced  by  the  living  cells  of  the 
plant  —  the  action  of  diastase  being  essentially  similar  to 
that  of  the  ptyalin  of  the  saliva.  The  germinating  plant 
thus  comes  to  contain  considerable  quantities  of  sugar,  to- 
gether with  salts,  proteins,  and  other  substances.  The 
watery  extract  of  malt  is  known  as  wort,  and  it  is  this 
which,  after  being  boiled  with  hops,  is  acted  upon  by  the 
yeast.  The  liquid  thus  produced  from  wort  by  fermentation 
is  known  as  ale,  beer,  stout,  porter,  etc.,  according  to  the 


294  THE  HUMAN  MECHANISM 

conditions  under  which  the  fermentation  takes  place  and 
the  character  of  the  malt  and  the  yeast  employed.  German 
beers  contain  from  three  to  four  per  cent  of  alcohol;  ale 
contains  from  four  to  six  per  cent. 

10.  Wines.    Wine  is  produced  by  the  fermentation  of  the 
juice  obtained  by  crushing  grapes,  and  the  yeast  comes  from 
the  "bloom"  on  the  skin  of  the  grapes.    The  juice,  or  "must," 
thus  extracted  is  allowed  to  undergo  fermentation,  and  the 
fermented  liquid  is   wine.     Most  wines,  however,   are   sub- 
jected to  subsequent  treatment.    Some  are  allowed  to  ripen 
in   wooden    casks,    during   which   process   there   take   place 
chemical  changes  which  give  to  each  wine  its  peculiar  flavor. 
In  other  cases  the  wine  is  "fortified"  by  the  direct  addition 
of  alcohol.    Wines  differ  from  one  another  according  to  the 
variety  of  the  grape  used  in  making  the  must,  according  to 
the  variety  of  yeast  used  for  fermentation,  and  according  to 
other  circumstances. 

11.  Distilled  liquors  and  spirits.    This  group  of  alcoholic 
beverages   contains   the  highest  percentage   of  alcohol,   and 
includes  whisky,  brandy,  rum,  and  gin.     In  the  making  of 
all  of  these  the   essential  procedure  is  the  same ;  namely, 
first   to  produce   fermentation   in   some    sugary  liquid   and 
afterwards  to  distill  from  the  products  of  this  fermentation 
its  alcohol  and  some  other  volatile  constituents.    Whisky  is 
made  by  distilling  fermented  corn  or  rye ;  brandy  may  be 
spoken  of  as  distilled  wine;  rum  is  distilled  from  fermented 
molasses,    and   gin  from   a  fermented   mixture   of   rye   and 
malt  —  juniper  berries  and  other  substances  being  added  to 
the   distilled   product.     In  general,  distilled  liquors  contain 
from  thirty  to  sixty  per  cent  of  alcohol. 

With  these  differences  of  preparation,  alcoholic  beverages 
differ  greatly  among  themselves,  independently  of  the  quan- 
tity of  alcohol  they  contain,  and  some  of  their  special  effects 
are  due  to  other  constituents.  The  chief  danger  of  most  of 


FOOD  ACCESSORIES  AND  DRUGS  295 

them,  however,  lies  in  the  action  of  the  ethyl  alcohol  upon 
the  system,  and  we  shall  confine  our  discussion  to  the  effects 
of  this  substance.  The  problem  is  by  no  means  a  simple 
one,  because  these  beverages  are  used  in  so  many  different 
ways  by  different  people.  Moreover,  the  results  of  their  use 
differ  according  to  the  constitution  of  the  person  using  them 
and  according  to  his  other  habits  of  life.  Sweeping  assertions 
are  too  frequently  made,  in  good  faith,  only  to  be  found  false 
by  experience  in  special  cases,  and  in  this  way  harm  is  done 
where  good  was  intended.  For  example,  it  is  often  asserted 
that  alcohol  used  in  any  amount  whatever  is  a  poison  to  the 
healthy  organism.  If  this  be  so,  it  is  the  only  known  drug 
of  which  this  is  true.  Dr.  John  J.  Abel,  from  whom  we 
shall  extensively  quote,  says  on  this  subject:  "All  poisons 
are  capable  of  being  taken  without  demonstrable  injury  in 
a  certain  quantity,  which  is  for  each  of  them  a  special 
though  sometimes  very  minute  fraction  of  their  toxic  or 
lethal  dose. ,  There  is  no  substance  which  is  always  and 
everywhere  a  poison."  Alcohol  is  a  drug  and,  like  many 
drugs,  may  be  and  frequently  is  used  in  poisonous  doses, 
but  it  must  not  be  supposed  that  its  real  danger  lies 
in  the  fact  that  it  always  exerts  a  poisonous  effect  on 
the  body. 

12.  The  physiological  action  of  alcohol.  As  to  the  imme- 
diate action  of  alcohol  on  the  body  we  may  say  that  it 
belongs  in  the  same  general  class  of  drugs  as  the  ether  and 
chloroform  used  for  anesthesia;  in  other  words,  its  general 
action  is  that  of  a  hypnotic  or  anesthetic.  To  quote  again 
from  Dr.  Abel: 

An  exhilarating  action  is  an  inherent  property  of  these  substances 
in  certain  doses.  Occasionally  the  physician  meets  with  persons  who 
have  formed  the  habit  of  inhaling  chloroform  from  the  palm  of  the 
hand  or  from  a  lightly  saturated  handkerchief.  The  inhalation  is 
usually  carried  on  for  a  short  time  only,  and  its  object  is  to  induce  a 
pleasant  form  of  mental  stimulation.  Only  occasionally  is  the  inhalation 
of  chloroform  carried  on  until  helpless  intoxication  occurs. 


296  THE  HUMAN  MECHANISM 

And  again: 

That  alcohol  can  produce  as  profound  anesthesia  as  any  of  the  sub- 
stances named  is  also  well  known.  In  the  days  before  anesthesia  it  was 
the  custom  of  bone  setters  to  ply  their  patients  with  alcohol  in  order  to 
facilitate  the  reduction  of  difficult  dislocations.  .  .  .  The  anesthesia  pro- 
duced by  alcohol  is,  however,  not  commendable,  since  it  cannot  safely 
be  induced  in  a  short  time  and  is  too  prolonged.  The  quantity  needed 
for  surgical  anesthesia  would  in  many  cases  lead  to  a  fatal  result. 

13.  Is  alcohol  a  stimulant?  The  view  of  the  action  of 
alcohol  just  stated  is,  of  course,  borne  out  by  the  condition 
of  a  thoroughly  intoxicated  person ;  but  it  is  opposed  to  the 
very  general  idea  that  alcohol,  except  in  large  doses,  is  to  be 
regarded  as  a  stimulant.  Whether  we  shall  call  it  a  *  stimu- 
lant" or  not  depends  upon  how  we  use  that  term.  Some  of 
the  exhilarating  effects  of  alcoholic  drinks  might  lead  us  to 
speak  of  it  hi  this  way.  People  who  have  drunk  wine  often 
become  more  talkative,  so  that  the  first  effects  of  intoxica- 
tion often  resemble  those  of  stimulation.  There  is,  however, 
strong  reason  for  thinking  that  this  action  is  only  super- 
ficially, and  not  fundamentally,  a  case  of  stimulation,  as  we 
shall  now  see. 

In  studying  the  physiology  of  the  nervous  system  we 
lound  that  processes  of  inhibition  are  as  important  in  its 
operation  as  are  those  of  excitation ;  and  in  mental  opera- 
tions the  course  of  our  thinking  is  constantly  checked  or 
inhibited  by  the  knowledge  of  facts  opposed  to  the  con- 
clusions towards  which  we  are  tending.  Probably  it  is  this 
essential  feature  of  all  accurate  and  valuable  mental  work 
which  is  the  first  to  be  paralyzed  by  alcohol.  The  man  who 
takes  alcohol  becomes  fluent  not  because  he  is  stimulated 
but  because  of  the  removal  of  checks  whose  presence  may 
make  him  talk  less  fluently,  but  which  at  the  same  time 
make  him  speak  more  accurately.  He  may  become  witty, 
and  may  say  some  brilliant  things,  but  he  will  almost 
always  do  and  say  some  very  erratic  things. 


FOOD  ACCESSOBIES  AND  DEUGS  297 

The  following  (by  Dr.  Abel)  appears  to  be  a  sound  state- 
ment of  our  present  knowledge  of  this  important  subject: 

Alcohol  is  not  found  by  psychologists  to  increase  the  quantity  or 
vigor  of  mental  operations  ;  in  fact,  it  clearly  tends  to  lessen  the  power  of 
clear  and  consecutive  reasoning.  In  many  respects  its  action  on  the  higher 
functions  of  the  mind  resembles  that  of  fatigue  of  the  brain,  though  with 
this  action  is  associated  a  tendency  to  greater  motor  energy  and  ease. 

In  speaking  of  a  certain  type  of  individual  James  says :  "  It  is  the 
absence  of  scruples,  of  consequences,  of  considerations,  the  extraordinary 
simplification  of  each  moment's  outlook,  that  gives  to  the  explosive  in- 
dividual such  motor  energy  and  ease."  This  description  aptly  applies 
to  the  individual  who  is  under  the  influence  of  a  "  moderate  "  quantity 
of  alcohol.  It  tends  to  turn  the  inhibitive  type  of  mind  into  the  "  hair- 
trigger  "  type.  We  have  said  that  the  speech  and  the  bearing  of  men, 
the  play  of  their  features,  all  bear  witness  to  the  action  of  alcohol  on 
the  brain ;  that  it  removes  restraints,  blunts  too  acute  sensibilities,  dis- 
pels sensations  of  fatigue,  causes  a  certain  type  of  ideas  and  mental 
images  to  follow  each  other  with  greater  rapidity,  and  gives  a  "cerebral 
sense  of  richness." 

Larger  quantities,  such  as  are  for  most  individuals  represented  by 
one  or  two  bottles  of  wine  (ten  per  cent  of  alcohol),  may,  according  to 
the  resistance  and  type  of  individual  in  question,  cause  a  lack  of  con- 
trol of  the  emotions ;  noticeably  affect  the  power  of  attention,  of  clear 
judgment  and  reason  ;  and  decidedly  lower  the  acuteness  of  the  several 
senses.  In  many  individuals  such  quantities  will  develop  so  marked  an 
anesthetic  action  that  all  phenomena  of  intoxication  may  be  seen  to 
follow  each  other  in  due  sequence,  finally  to  end  in  the  sleep  of 
drunkenness. 

There  has  been  much  discussion  as  to  whether  alcohol  is  in  any 
sense  a  stimulant  for  the  brain.  We  have  seen  that  pharmacologists  of 
high  repute  deny  that  it  has  this  action,  holding  that  alcohol  is  a  seda- 
tive or  narcotic  substance  which  belongs  to  the  same  class  as  paralde- 
hyde  and  chloroform  ;  that  its  stimulating  action  is  but  fictitious ;  and 
that  even  the  earlier  phenomena  of  its  action  are  to  be  referred  to  a 
paralyzing  action  on  cerebral  (inhibitory)  functions.  This  theory  as- 
sumes an  unequal  action  on  cerebral  functions  in  the  order  of  time. 
Kraepelin,  however,  holds  that  this  is  a  purely  subjective  analysis,  and 
that  in  the  early  stages  of  its  action  alcohol  truly  stimulates  the  motor 
functions  of  the  brain ;  that  a  state  of  mental  exhilaration,  of  "  motor 
excitability,"  may  coexist  with  undiminished  power  of  perception  and 
judgment.  His  psychological  experiments  on  the  action  of  alcohol, 
taken  all  in  all,  do  not,  however,  entirely  prove  his  position. 

p 


298  THE  HUMAN  MECHANISM 

Some  cases  of  apparent  stimulation  are  really  due  to  the 
fact  that  alcohol,  when  taken  in  the  form  of  wines  and  dis- 
tilled liquors,  sets  up  an  irritation  in  the  mucous  membrane 
of  the  mouth,  oesophagus,  and  stomach,  which  reflexly  excites 
the  heart  to  greater  activity  or  for  the  time  being  reflexly 
stimulates  the  nervous  system.  Such  stimulation  is,  however, 
transient  and,  as  the  alcohol  is  absorbed  into  the  blood,  gives 
way  to  depression  and  even  stupor. 

It  is  neither  possible  nor  necessary  to  state  here  in  full 
the  reasons  which  have  led  to  what  seems  to  the  authors 
the  erroneous  view  that  alcohol  in  small  doses  is  a -stimu- 
lant and  only  in  larger  doses  a  depressant  and  hypnotic. 
Enough  has  been  said  to  show  that  there  are  at  least  two 
opinions  about  the  matter:  that  even  if  alcohol  is  at  times 
a  stimulant,  it  is  an  uncertain  stimulant,  and  that  its  excita- 
tion is  liable  to  give  way  at  any  time  to  depressing  effects. 
A  critical  examination  of  the  literature  on  the  subject  has 
failed  to  demonstrate  to  us  a  direct  stimulating  action  of 
alcohol  on  any  of  the  functions,  such  as  the  beat  of  the 
heart,  respiration,  digestion,  etc.  At  tunes,  especially  in  sick- 
ness, alcohol  may  be  useful;  but  the  evidence  tends  to  the 
conclusion  that  where  it  exerts  any  physiological  action 
on  the  healthy  body  at  all,  that  action  is  usually  depress- 
ing. This  is  notably  true  as  to  the  beat  of  the  heart,  as  to 
respiration,  and  as  to  the  ability  to  do  muscular  work. 

We  have  dwelt  at  length  upon  this  question  in  order  to 
disabuse  the  student's  mind  of  the  idea  that  alcoholic  drinks 
can  be  safely  depended  upon  as  an  aid  in  the  performance 
of  work.  Few  causes  are  more  effective  in  leading  to  the 
abuse  of  alcohol  than  the  idea  that  when  one  finds  difficulty 
in  doing  a  thing  it  may  be  accomplished  more  easily  by 
having  recourse  to  beer  or  wine  or  whisky  for  their  "  stimu- 
lating" effect.  In  general,  so  far  is  this  from  being  the 
truth  that  the  person  seeking  such  aid  is  really  using  a  hyp- 
notic and  depressant.  Obviously  he  would  be  acting  more 


FOOD  ACCESSORIES  AND  DRUGS  299 

wisely  to  adopt  other  methods  of  accomplishing  his  end. 
Nor  is  this  conclusion  merely  theoretical.  Brain  workers 
who  wish  to  "  keep  a  clear  head  "  almost  universally  avoid 
alcoholic  drinks,  at  least  until  work  is  over. 

14.  Alcohol  in  muscular  work.    That  the  general  effect  of 
alcoholic    drinks   is   to   depress    rather    than   stimulate    the 
powers  of  the  body  is  furthermore  indicated  by  the  results 
of  experiments  on  men  doing  heavy  work,  as,  for  example, 
soldiers  on  forced  marches.    In  the  Ashanti  campaign  the 
effect  of  alcohol  as  compared  with  beef  tea  was  tested.    To 
quote  from  Sir  Lauder  Brunton : 

It  was  found  that  when  a  ration  of  rum  was  served  out,  the  soldier 
at  first  marched  more  briskly,  but  after  about  three  miles  had  been 
traversed  the  effect  of  it  seemed  to  be  worn  off,  and  then  he  lagged 
more  than  before.  If  a  second  ration  were  given,  its  effect  was  less 
marked,  and  wore  off  sooner  than  that  of  the  first.  A  ration  of  beef  tea, 
however,  seemed  to  have  as  great  a  stimulating  power  as  one  of  rum,  and 
not  to  be  followed  by  any  secondary  depression. 

The  results  of  these  and  other  experiments  lead  us  to 
the  conclusion  that  alcohol  cannot  be  depended  upon  to  in- 
crease the  capacity  for  hard  muscular  work  and  that  in  the 
great  majority  of  cases  it  actually  diminishes  it. 

15.  The  dilation  of  cutaneous  arteries  by  alcohol.    One  of 
the  most  important  effects  of  alcoholic  drinks  is  the  dilation 
of  the  arteries  of  the  skin,  thus  sending  more  warm  blood 
to  the  surface.    It  is  a  common  experience  among  persons 
not  accustomed  to  alcoholic  drinks  that  even  a  small  amount 
"  makes  the  face  hot "  and  flushed,  and  the  red  face  of  the 
toper  is  proverbial.    The  result  of  this  dilating  effect  is  that 
the  temperature  of  the  skin  rises  and  the  individual  feels 
warmer.    Congested  states  of  internal  organs  may  thus  be  re- 
lieved, and  this  is  probably  one  reason  why  men  leading  an 
exclusively  sedentary  life  often  use  alcoholic  drinks  apparently 
to  some  advantage.   But  even  these  would  do  infinitely  better 
to  secure  the  same  result  by  proper  muscular  activity. 


300  THE  HUMAN  MECHANISM 

Even  if  a  temporary  advantage  appears  to  be  gained 
in  some  cases  or  at  some  times,  this  has  often  to  be 
paid  for  by  bad  secondary  effects,  such  as  impaired  ca- 
pacity for  good  work  some  hours  later;  and  in  mental 
work  of  the  highest  kind,  such  as  original  writing  or 
composition,  the  after  effects  of  alcoholic  drinks  are  some- 
times prolonged  and  easily  detected  by  the  subject  of 
the  experiment. 

16.  Alcohol  as  a  defense  against  exposure  to  cold.  Because 
of  this  effect  upon  the  cutaneous  circulation  alcoholic  drinks 
are  frequently  used  by  men  exposed  to  cold,  with  the  mis- 
taken idea  that  the  conditions  within  the  body  are  thereby 
improved.  The  student  has,  however,  learned  (p.  193)  that 
a  feeling  or  sensation  of  warmth  does  not  necessarily  indicate 
greater  heat  production  within  the  body;  and  he  also  knows 
that  bringing  the  blood  to  the  skin  when  the  body  is  ex- 
posed to  cold  serves  to  increase  the  loss  of  heat.  As  a 
matter  of  fact  the  internal  temperature  often  falls  when 
alcohol  is  taken  under  these  conditions.  The  story  is  told 
of  some  woodsmen  who  were  overtaken  by  a  severe  snow- 
storm and  had  to  spend  the  night  away  from  camp;  they 
had  with  them  a  bottle  of  whisky,  and,  chilled  to  the  bone, 
some  imbibed  freely,  while  others  refused  to  drink.  Those 
who  drank  soon  felt  comfortable  and  went  to  sleep  in  their 
improvised  shelter;  those  who  did  not  drink  felt  very  un- 
comfortable throughout  the  night  and  could  get  no  sleep, 
but  in  the  morning  they  were  alive  and  able  to  struggle 
back  to  camp,  while  their  companions  who  had  used  alco- 
holic drinks  were  found  frozen  to  death.  They  had  pur- 
chased relief  from  their  unpleasant  sensations  of  cold  at  the 
cost  of  lowering  their  body  temperature  below  the  safety 
point.  This,  if  true,  was,  of  course,  an  extreme  case;  but 
it  accords  with  the  universal  experience  of  arctic  travelers 
and  of  lumbermen  and  hunters  in  northern  woods,  that  the 
use  of  alcohol  during  exposure  to  cold,  although  contributing 


FOOD  ACCESSOEIES  AND  DKUGS  301 

greatly  to   one's   comfort  for  the   time   being,   is   generally 
followed  by  undesirable  or  dangerous  after  effects. 

17.  Alcohol  as   a  food.    There  has  been  much  discussion 
as  to  whether  alcohol  is  or  is  not  a  food;  that  is,  whether 
its    oxidation    within   the   body   may    supply    energy.     This 
question  must  now  be  answered  in  the  affirmative,  although 
whether  it   can  do  more  than  supply  heat  to  maintain  the 
body  temperature,  —  that  is,  whether  it  can  also  supply  the 
power  for  muscular  work,  as  do  fats  and  carbohydrates,  —  we 
cannot  in  the  present  state  of  our  knowledge  positively  say. 
In  many  cases  of  sickness  the  oxidation  of  alcohol  is  prob- 
ably a  useful  source  of  heat  production,  since  it  is  absorbed 
quickly  and  without  digestion,  but  the  healthy  man   does 
not  and  should  not  use  it  in  this  way.    The  amounts  which 
would  be  required  to  be  of  any  considerable  service  as  food 
are  far  beyond  those  in  which  it  may  be  used  with  safety. 
In  other  words,  in  using  alcohol  for  food  one  would  be  ob- 
taining heat  at  the  cost  of  direct  injury  to  many  organs  and 
also  at  the  cost  of  impaired  working  power.    Moreover,  men 
do  not  use  alcohol  as  a  food ;  they  use  it  as  a  drug.    So  that 
while  the  action  of  alcohol  as  a  food  is  of  practical  impor- 
tance  to   the   physician,  who  must  deal  with  the  abnormal 
conditions  of  disease,  its  action  as  a  food  is  not  a  matter  of 
practical  importance  to  healthy  people. 

18.  Pathological    conditions    due    to    the    use    of    alcohol. 
When   alcoholic  beverages   are  taken  in  excessive  amounts 
we  have  the  sad  and   degrading  spectacle   of  a   "  drunken 
spree."    Whether  or  not  the  drinker  at  first  appears  bright 
or  witty,  sooner  or  later  there  is  presented  the  pitiable  pic- 
ture of  complete  loss  of  nervous  coordination   and  control. 
The  man  becomes  silly,  or  maudlin,  or  pugnacious,  as  the 
case   may  be,  but  always  irrational;  he  staggers,  stumbles, 
or  falls ;  and  finally  passes  into  a  drunken  stupor.    In  this 
event    the    victim    of    his    own    indulgence    is    said  to    be 
"dead"  drunk,  or  "intoxicated,"  being  as  it  were  thoroughly 


302  THE  HUMAN  MECHANISM 

poisoned.  If  such  intoxication  is  frequently  repeated,  there 
is  a  complete  breakdown  of  the  nervous  system ;  the  victim 
of  alcoholic  indulgence  becomes  a  raving  maniac  and,  with 
disordered  vision,  thinks  he  sees  all  about  him  snakes  or 
foul  vermin  (delirium  tr emeus).  The  silly  or  foolish  stage 
of  this  poisoning  sometimes  provokes  smiles  or  laughter  in 
thoughtless  observers,  but  none  can  witness  the  more  serious 
consequences  of  repeated  intoxication  by  alcoholic  drinks 
without  disgust  and  horror. 

Many  steady  drinkers,  even  though  they  have  never  been 
drunk  in  their  lives,  are  apt  ultimately  to  acquire  various 
diseased  conditions  of  the  body,  into  which  we  cannot  enter 
in  detail.  The  heart  may  be  injured,  or  the  arteries  become 
diseased ;  the  repeated  irritation  of  the  stomach  may  produce 
chronic  gastritis ;  or  the  connective  tissue  of  the  liver  and 
kidneys  may  increase,  thus  crowding  upon  the  living  cells 
and  ultimately  throwing  a  large  part  of  them  entirely  out  of 
use.  While  it  must  not  be  supposed  that  drinking  alcohol  is 
the  sole  cause  of  these  troubles, — for  some  or  all  of  them 
may  come  from  other  causes, — the  frequency  of  their  occur- 
rence in  steady  drinkers  is  suspiciously  high,  and  this  has 
led  to  the  very  strong  conviction  among  medical  men  that 
alcohol  plays  a  large  role  in  producing  them. 

19.  Summary  of  the  action  of  alcohol  as  a  drug.  In  small 
doses  alcohol  may  be  completely  oxidized  within  the  body 
without  exerting  any  pharmacological  action.  In  the  forms 
and  amounts  usually  employed  in  alcoholic  beverages  it 
exerts,  in  general,  a  hypnotic  or  anesthetic  action ;  the  result 
on  the  system  as  a  whole  depends  on  the  amount  taken, 
and  varies  from  the  paralysis  of  inhibitory  processes  to  the 
depression  of  all  nervous  functions,  ending  in  drunken 
stupor.  Continued  excess  may  produce  exaggerated  forms 
of  temporary  insanity,  among  which  delirium  tremens  may 
be  mentioned.  There  is,  moreover,  good  reason  for  believing 
that  steady  drinking  is  very  frequently  an  important  agent 


FOOD  ACCESSORIES  AOT>  DRUGS  303 

in  preparing  the  way  for  many  other  diseases,  and  is  hence 
a  serious  menace  to  health. 

20.  The  seat  of  the  danger  in  alcoholic  drink.  The  regular 
use  of  alcoholic  beverages  is  dangerous  for  the  same  reason 
that  the  regular  use  of  any  drug  is  dangerous.  We  are  too 
apt  to  rely  upon  the  drug  to  do  for  us  what  we  ought  to 
accomplish  only  by  the  hygienic  conduct  of  life ;  the  drug 
never  satisfactorily  does  the  work,  and  we  go  from  bad  to 
worse,  and  become  its  slave.  But  there  is  certainly  greater 
danger  in  hypnotic  drugs,  like  alcohol,  than  in  true  stimu- 
lants, like  coffee,  and  cocoa,  and  tea.  We  need  to  have 
ourselves  well  under  control  when  we  use  any  drug;  the 
highest  faculties  of  the  mind  must  keep  tight  rein  or  we 
may  lose  control  of  ourselves.  With  hypnotic  drugs  —  to 
which  class  belong  not  only  alcohol  but  ether,  chloroform, 
opium,  chloral,  etc.  —  there  is  special  danger  that  these 
powers  of  control  (inhibition)  may  be  stealthily  paralyzed 
before  we  know  it.  Of  course  thousands  of  people  use 
alcohol  in  moderation  and  never  become  drunkards ;  but 
thousands  also,  with  no  intention  of  using  it  to  excess,  do 
unconsciously  let  the  reins  drop,  and  before  they  know  it 
the  drug  gets  the  better  of  them.  Experience  shows  that 
it  is  with  the  hypnotic  drugs  that  this  most  frequently 
happens. 

Again,  if  we  make  a  habit  of  taking  alcoholic  drinks,  we 
are  specially  exposed  to  temptation  from  our  fellow  men  to 
go  too  far.  For  the  most  part,  people  take  coffee  and  tea 
or  do  not  take  them,  as  they  please ;  no  one  urges  them  to 
use  these  drinks  when  they  are  disinclined  to  do  so.  To  a 
less  degree  the  same  thing  is  true  of  tobacco,  although  here 
the  force  of  fashion  and  example  is  stronger.  But  with 
alcoholic  beverages  the  custom  of  "  treating "  makes  the 
exercise  of  self-restraint  more  difficult  than  it  would  other- 
wise be,  for  here  we  are  dealing  with  a  drug  which  is 
capable  of  impairing  self-control.  Some  one  "treats"  a  friend 


304  THE  HUMAN  MECHANISM 

to  a  drink;  the  friend  wishes  to  return  the  compliment  and 
so  they  drink  again;  the  person  with  deficient  self-control — 
and  what  little  he  has  now  lessened  —  insists  upon  a  third, 
and  so  on,  perhaps  to  intoxication.  This,  of  course,  does 
not  always  happen;  thousands  are  strong  and  escape  the 
danger,  but  thousands  are  weak  or  do  not  know  better,  and 
many  a  week's  wages  has  gone  in  this  way,  leaving  behind 
poverty  and  misery  and  impaired  capacity  before  the  close 
of  Saturday  night. 

21.  Concluding  remarks  on  the  use  of  alcoholic  beverages. 
In   the   foregoing   pages   we  have   stated   the    salient   facts 
concerning  the  physiological  action  of  alcohol  and  alcoholic 
drinks.    It   only  remains  to  point  out  for  the  student  the 
obvious  conclusions  to  be  drawn  from  them  and  from  the 
long  and,  on  the  whole,  very  sad  experience  of  the  race  with 
alcoholic  drinks.     The  first  is  that   except  in  sickness  and 
under  the  advice  of  a  physician,  alcoholic  drinks  are  wholly 
unnecessary  and  much  more  likely  to  prove  harmful  than 
beneficial.    The  second  is  that  their  frequent  and  especially 
their  constant  use  is  attended  with  the  gravest  danger  to 
the  user,  no  matter  how  strong  or  self-controlled  he  may  be. 

It  is  true  that  history  and  romance  and  poetry  contain 
many  attractive  allusions  to  wine  and  other  alcoholic  drinks, 
and  it  may  also  be  true  that  such  drinks,  by  loosening 
tongues  and  breaking  down  social,  political,  or  other  barriers 
(removing  inhibitions),  may  tend  towards  conviviality  and 
good-fellowship ;  but  it  is  no  less  true  that  the  path  of  his- 
tory is  strewn  with  human  wreckage  directly  due  to  alcohol; 
that  many  a  promising  career  has  been  drowned  in  wine; 
and  that  indescribable  misery  follows  in  the  trail  of  drunken- 
ness. The  only  absolutely  safe  attitude  toward  alcoholic 
drinks  is  that  of  total  abstinence  from  their  use  as  beverages. 

22.  Opium,  morphine,  and  the  opium  habit.    The  danger 
of  the  use   of   drugs  as   a  regular  habit  of  life  is  perhaps 
most  painfully  illustrated  by  what  is  known  as  the  opium 


FOOD  ACCESSORIES  AND  DRUGS  305 

habit.  Among  the  most  valuable  remedies  at  the  physician's 
disposal  is  opium  or  its  active  principle,  •morphine,  which 
possesses  remarkable  power  to  produce  insensibility  to  pain. 
It  sometimes  happens,  however,  that  by  incautiously  using 
this  drug  for  this  purpose  men  and  women  become  addicted 
to  the  habit.  They  finally  cannot  do  without  the  drug,  and 
its  constant  use  causes  an  appalling  moral  and  physical  de- 
generation ;  so  far  indeed  does  this  often  go  that  the  victim 
will  commit  crime  in  order  to  obtain  the  drug.  It  should 
be  clearly  understood  that  it  is  unsafe  for  anyone  to  use 
opiates  to  relieve  pain;  indeed,  these  should  never  be  used 
except  when  prescribed  by  a  careful  physician. 

23.  Chloral,  cocaine,  etc.    Men  and  women  may  become 
slaves  to    the  use  of   other  drugs   and  in  much  the   same 
way  as  they  become  slaves  to  alcohol  and  morphine.    Among 
these   drugs  are  chloral   and   cocaine.    They  belong   in  the 
same   general   group    of   hypnotics   or   anesthetics,    and    the 
habit  acquired  is  perhaps  no  worse  than  the   opium  habit. 
It  is  certainly  very  little  better.    Let  the  student  remember 
that  the  root  of  the  evil  here,  as  elsewhere,  is  the  substitution 
of  the  use  of  the  drug  for  normal  habits  of  healthful  living. 

24.  Tobacco.   The  physiological  effects  of  tobacco  are  quite 
complicated,    so   complicated    that    it    is    difficult    to    make 
general   statements    with   regard    to    them.     The    effects    of 
chewing    are    quite    different   from   those    of   smoking,    and 
those  of  smoking,  no  doubt,  vary  according  as  the  smoke 
is  or  is  not  drawn  into  the  lungs  (inhaled). 

The  leaf  of  tobacco  contains  a  poison  (nicotine)  which 
exerts  a  powerful  action  on  the  heart  and  on  nerve  cells. 
It  is  not,  however,  proved  that  the  bad  effects  of  the  use 
of  tobacco  are  due  entirely  or  even  chiefly  to  this  substance, 
but  it  unquestionably  contributes  to  the  physiological  effects. 

The  smoke  from  tobacco  also  contains  ammonia  vapor 
which  locally  irritates  the  mucous  membrane  of  the  mouth, 
throat,  nose,  etc.,  and  this  irritating  action  at  times  acts. 


306  THE  HUMAN  MECHANISM 

as  a  stimulant  to  the  whole  system  in  much  the  same 
manner  as  do  "  smelling  salts." 

It  has  been  recently  suggested  that,  owing  to  the  incom- 
plete character  of  the  combustion,  tobacco  smoke  contains 
a  small  amount  of  the  poisonous  gas  carbon  monoxide  (CO), 
and  it  is  quite  possible  that  some  effects  of  smoking  — 
especially  where  the  smoke  is  drawn  into  the  lungs  (inhaled) 
—  may  be  attributed  to  this  gas;  but  the  suggestion  has 
not  yet  been  submitted  to  the  test  of  actual  experiment. 

Indeed,  the  physiological  action  of  tobacco  probably  not 
only  varies  with  the  form  in  which  the  tobacco  is  used  but 
is  in  any  case  the  result  of  a  combination  of  a  number  of 
factors  partly  physiological  and  partly  psychical.  We  must 
here,  however,  confine  our  attention  to  the  purely  hygienic 
aspects  of  the  matter. 

Human  experience  shows  that  the  unwise  use  of  tobacco 
may  unfavorably  affect  digestion,  cause  serious  disorders  of 
the  heart,  and  impair  the  work  of  the  nervous  system.  Those 
training  for  athletic  events  are  usually  forbidden  the  use  of 
tobacco  because  it  "  takes  the  wind  "  ;  that  is,  makes  impossi- 
ble the  most  efficient  training  of  the  heart.  Many  employers 
have  found  that  youths  who  smoke  cigarettes  are  less  reliable 
in  their  work ;  and  this  is  only  one  instance  of  the  effect 
upon  the  nervous  system  already  referred  to,  the  same  result 
being  observed  in  a  diminished  steadiness  of  the  hand,  often 
amounting  to  actual  tremor. 

These  effects  do  not,  of  course,  manifest  themselves  in 
their  extreme  form  whenever  tobacco  is  used,  but  it  is 
probable  that  they  are  always  present  in  some  degree. 
Whether  they  are  noticeable  or  not  depends  largely  upon 
the  ability  of  the  constitution  to  resist  them.  Tobacco  is 
thus  often  used  without  demonstrable  bad  effects  when  one 
is  leading  a  hygienic  life ;  but  very  often  the  habit,  formed 
under  these  conditions,  persists  after  the  increasing  intensity 
of  occupation  and  the  attendant  cares  and  responsibilities 


FOOD  ACCESSORIES  AND  DRUGS  307 

of  life  result  in  neglect  of  muscular  exercise  and  improperly 
directed  nervous  activity.  As  this  neglect  begins  to  tell  on 
general  health  it  is  found  that  the  unfavorable  effects  of 
tobacco  become  more  pronounced. 

Especially  to  be  condemned  is  its  use  by  those  who  have 
not  attained  their  full  growth.  During  youth  nothing  should 
be  allowed  to  interfere  with  the  best  development  of  the 
heart  and  nervous  system,  and  the  use  of  tobacco  endangers 
the  proper  development  of  both  of  these  most  important 
parts  of  the  human  mechanism.  It  can  hardly  be  doubted 
that  many  a  young  man  has  failed  to  make  the  most  out  of 
life  because  the  habit  contracted  in  youth  has  struck  in  this 
way  at  the  foundations  upon  which  he  had  subsequently 
to  build. 


FIG,  154.  The  thoracic  and  abdominal  cavities,  after  the  removal  of 
the  organs  shown  in  Fig.  2 

The  diaphragm  has  been  drawn  somewhat  forward 


I  — 


FIG.  155.  General  view  of  the  digestive  tract.    After  Spalteholz 

A,  mouth  cavity ;  B,  pharynx;  (7,  oesophagus ;  D,  diaphragm;  E,  stomach; 
FI  small  intestine ;  G,  ascending  colon ;  H,  descending  colon ;  /,  rectum. 
The  transverse  colon  has  been  cut  away,  its  position  being  indicated  by 
dotted  lines 


FIG.  156.  The  flouncelike  folding  of  the  mesentery,  as  seen  after 
removing  the  small  intestine.    After  Spalteholz 


•TV*  •"•  "•  •  i  *. 
:*  •••" :'•":"•:  :/•: 


FIG.  157.  Median  dorso-ventral  section  of  the  trunk  in  the  abdominal 
region,  showing  the  suspension  of  the  stomach  and  intestine  by  the 
mesentery.  After  Spalteholz 

A,  liver;  B,  stomach;  C,  transverse  colon;  D,  mesentery;  E,  rectum; 
r  F,  urinary  bladder 


FIG.  158.  The  permanent  teeth  in  the  jaw-bones,  viewed 
from  the  right.    After  Spalteholz 


FIG.  159.  The  network  of  capillaries  on  the  lining  of  the 
air  cells  of  the  lungs.    After  Kolliker 

See  page  169 


A—  4 


B 


FIG.  160.  First  layer  of  muscles  of  the  breast  and  shoulder  region. 
After  Spalteholz 

A,  biceps  of  the  arm  (p.  33) ;  B,  deltoid;  C,  portion  of  the  trapezius  (see 
Figs.  113  and  114);  D,  clavicle;  E,  sternum  or  breastbone;  F,  pecto- 
ralis  major  (see  p.  316  and  Fig.  114) 


I) 


— H 


—  K 


FIG.  161.  Second  layer  of  muscles  of  the  breast,  exposed  by  dissecting 
away  the  pectoralis  major  in  Fig.  160.    After  Spalteholz 

A,  B,  the  two  "  heads  "  of  the  biceps  ;  C,  cut  end  of  the  pectoralis  major ; 
D,  deltoid ;  E,  pectoralis  minor ;  F,  trapezius ;  G,  clavicle ;  H,  first  rib ; 
X,  sternum.  Note  the  direct  attachment  of  the  intercostal  muscles  to 
the  ribs  (p.  8).  Compare  Fig.  160 


S 


«  'C  73    O 

£  «*  .H  "* 
a  &*!, 

•"!-  -2  "£  co 

fcfi   K     >*•     C-i 


III 


o 

£>  M 

f~^.  O 

®  "^ 

-Q  «M 

Id 

M-l  O 

0  fe 


OD    N 
~    'O 

II 


|| 


The  parts  of  the  nervous  system 
represented  are  the  cerebrum, 
cerebellum,bulb,and  segment 
of  the  spinal  cord.    Afferent 
nerves  in  red,  efferent  nerves 
in  black,    m,  m,  motor  neu- 
rones to  some  of  the  muscles 
of  the  leg.     These   may  be 
stimulated  to  coordinate  ac- 
tion   by   neurones  (v)  from 
the  cerebrum,  neurones  (cb) 
from  the  cerebellum,  or  by 
the  afferent  neurones  (a/1) 
from  the  tendons,  etc.    In  the 
bulb  this   afferent    neurone 
connects  with  a  second  neu- 
rone (a/2),  and  this  with  a 
third  (a/3),    thus  providing 
the  path  to  the  cerebrum  and  exciting 
in  consciousness  sensations  of  position 
of  the  leg  (muscular  sense).    The  same 
neurones  connect  with  the  cerebellum, 
as  do  also  neurones  from  the  inner  ear. 
For  further  explanations  see  Chapter 
XV,  pp.  275-279. 


Dia^am  jTfJtlie  Nervous  mechanism  of  walking 


FIG.  166.  Side  view  of  the  brains;  of;  i^bfc£t,\c 
See  pa?e  267  " 


INDEX 


Abdominal  breathing,  172 

Abdominal  cavity.  See  Peritoneal 
cavity 

Abdominal  muscles,  action  of,  in 
breathing,  174 

Absorption  from  the  intestine,  126 

Accommodation,  in  vision,  for  near 
objects,  243  ;  muscle  of,  244  (fig.) 

Adipose  tissue,  184,  222 

Adrenal  glands,  64 

Adrenalin,  action  of,  64,  163 

Aerial  blanket,  198 

Afferent  impulses,  76  ff . ;  reflex  and 
conscious  effects  of,  275  ff. 

Afferent  neurones,  76,  78 

Air,  stagnant,  198 

Air  cell,  169 

Albuminoids,  94 

Alcohol,  physiological  action  of, 
295  ;  as  a  stimulant,  296  ;  in  mus- 
cular work,  299 ;  as  a  defense 
against  cold,  300 ;  as  a  food,  301  ; 
pathological  conditions  due  to, 
301 ;  influence  on  self-control, 
303-304 

Alcoholic  beverages,  composition  of, 
292 

Alimentary  canal,  structure  of,  20, 
107,  118,  128 

Alimentation,  91,  98 

Alveolus  of  gland,  30,  31  (fig.),  32 
(fig.) ;  of  lungs,  169,  170  (fig.) 

Amino-acids,  102 

Amoeba,  amoeboid  movement,  137 

Arnylopsin,  120 

Anesthetics,  280 

Animal  foods,  97,  111 

Ankle,  bones  of,  19 

Anterior,  definition  of  term,  9 

Aorta,  13,  22,  23  (fig.),  25,  145 

Apical  lobes  of  lungs.    See  Lungs 

Appendicular  skeleton,  18 

Aqueduct  of  Sylvius,  265 

Aqueous  humor,,  243 

Arborization.    See  Synapse 

Arterial  reservoir,  142 


Arterial  tone,  161 
Arteries,  21,  37,  148  (fig.) 
Astigmatism,  249 
Auditory  nerve,  27,  256 
Augmentor  nerves  of  heart,  160 
Auricle,  21,  23  (fig.),  140 
Auriculo-ventricular  valves,  141 
Automatic  nervous  actions,  81 
Axial  skeleton,  14 
Axon,  or  axis  cylinder,  72,  75 

Bile,  121 

Bile  duct,  108  (fig.) 

Bladder,  urinary,  181,  182  (fig.) 

Blood,  arterial  and  venous,  24,  166 ; 
as  a  common  carrier,  135  ;  micro- 
scopic structure  of,  136  ;  distribu- 
tion among  organs,  146 ;  gases  of, 
166 

Blood  corpuscles,  red,  136-138 ;  as 
carriers  of  oxygen,  167 

Blood  corpuscles,  white,  136-137 

Blood  plasma,  138  ;  gases  of,  166 

Blood  vessels,  37.  See  also  Arteries, 
Capillaries,  and  Veins 

Body  cavity,  10 

Bone,  36.    See  also  Skeleton 

Brain,  27 ;  the  seat  of  sensations, 
241  ;  of  frog,  264  ;  of  mammal, 
265 ;  histological  structure  of,  268 ; 
functions  of,  270  ff. 

Breastbone,  17 

Breathing  movements,  171  ff. ;  effect 
on  circulation,  148,  175  ;  effect  on 
flow  of  lymph,  150,  175 ;  hygiene 
of,  174 

Breathlessness,  178 

Bronchiole,  170  (fig.) 

Bronchus,  12,  21,  169 

Bulb,  264  (fig.),  265,  267  (fig.); 
functions  of,  272 

Caffeine,  290 

Calorie,  216 

Canal,  spinal,  or  vertebral,  17 

Capillaries,  24  (fig.),  27,  31  (fig.),  37 


309 


310 


ELEMENTS  OF  PHYSIOLOGY 


Capsule  of  gland,  30 

Carbohydrates,  94;  digestion  of, 
104,  112,  120 ;  as  source  of  power 
for  work,  217  ff. ;  fuel  value  of, 
217;  as  food  in  cold  climates, 
220 ;  as  source  of  fat,  224 

Carbon  dioxide  (carbonic  acid), 
formed  during  muscular  work, 
48,  60,  178,  299;  in  lymph,  167; 
in  blood  plasma,  168  ;  action  of, 
on  respiratory  center,  176 

Cardiac  region  of  stomach,  107 

Cell  walls  in  plants,  98 

Cells,  31,  32,  34,  36,  39,  40  (fig.),  41 ; 
as  chemical  factories,  46  ff . ;  waste 
and  repair  of,  229  ff. 

Cellulose,  97 

Central  canal  of  spinal  cord,  264 

Cerebellum,  264  (fig.),  265,  267,  270 
(fig.),  279 

Cerebrum,  82,  264  (fig.),  266  (fig.), 
267  (fig.),  268  (fig.);  connections 
with  other  parts  of  the  nervous 
system,  275  ;  functions  of,  279  ff. 

Cervical  vertebrae,  14 

Chloral,  305 

Chloroform,  280 

Chocolate,  291 

Choroid,  243,  244,  247  (fig.) 

Chyme,  117 

Ciliary  muscle,  244 

Ciliary  region  of  eye,  244  (fig.) 

Cinders  removed  from  eye,  394 

Circulation,  organs  of,  21,  143  (fig.) ; 
time  of,  136 ;  mechanics  of,  139  ; 
in  warm  and  cold  weather,  152, 
201  ;  during  muscular  activity, 
154 ;  during  mental  work  and 
sleep,  155 ;  during  digestion,  157, 
158  ;  nervous  factors  of,  159  ff. ; 
essential  to  respiration,  177 

Clavicle,  19 

Cleavage,  chemical,  in  muscular  con- 
traction, 50  ;  of  starch  and  protein 
in  digestion,  102 

Climate,  and  mental  work,  207 ;  and 
feeding,  220 

Coagulation  of  proteins,  93 

Cocaine,  305 

Coccygeal  vertebrae,  14 

Cochlea,  257 

Cocoa,  291 

Coffee,  290 

Cold,  effect  on  circulation  of  the 
blood,  152 ;  effect  on  body  as 


a  whole,  201  ff. ;  sensations  of, 
258 

Cold-blooded  animals,  191 

Collagen,  94 

Collar  bone.    See  Clavicle 

Collaterals,  78 

Colon,  20 

Color,  sensations  of,  252 

Compensatory  adjustments  of  the 
circulation,  153 

Conditioned  reflexes,  85 

Conduction  of  heat,  211 

Connective  tissues,  structure  of,  7,  8, 
37,  222  ;  of  glands,  30,  31  ;  of 
muscles,  33-34  ;  relation  of,  to 
blood  vessels,  36 ;  relation  of,  to 
lymphatics,  39  ;  of  nerves,  71 ;  di- 
gestion of,  110;  of  lungs,  169;  of 
skin,  184 

Consciousness,  240,  273,  279 

Constant  temperature  of  the  body, 
190  ;  maintenance  of,  201  ff. 

Constipation,  132 

Consumption.    See  Tuberculosis 

Contagious  diseases.    See  Diseases 

Contraction  of  musclB,  33,  47,  56 

Convection  of  heat,  212 

Convolutions  of  cerebrum,  266  (fig.), 
267 

Coordination,  70,  83,  271  ;  training 
in,  87 , 283 ;  by  chemical  means,  89 

Corium,  184 

Cornea,  243,  244  (fig.),  247  (fig.) 

Corpuscles.    See  Blood  corpuscles 

Cortex  of  cerebrum  and  cerebellum, 
269 

Costal  breathing,  172 

Cranial  nerves,  268 

Cranium,  17 

Curd  of  milk,  93 

Curvatures  of  vertebral  column, 
14  ff.,  321 

Curve  of  fatigue,  56 

Cutaneous  sensations,  258 

Cutis,  184 

Cytoplasm,  31,  35,  41,  75 

"  Danger  zone  "  of  atmospheric  tem- 
perature, 205 

Dendrites,  75,  268,  270,  271 

Dermis,  184 

Dextrines,  101  (fig.),  105 

Dextrose,  101 

Diaphragm,  10 ;  action  in  respira- 
tion, 171-173 


INDEX 


311 


Diarrhea,  133 

Diastase,  293 

Diastole,  140,  142  (fig.) 

Diet,  value  of  a  mixed,  231 

Digestion,  organs  of,  20  ;  nature  of, 
100 ;  external  and  internal,  100  ; 
in  the  mouth,  103  ;  in  the  stomach, 
107;  in  the  intestine,  117;  sum- 
mary of  chemical  processes  of, 
123  ;  cooperation  of  processes  of, 
132 ;  and  the  circulation,  157 ; 
and  temperature  regulation,  207 

Distilled  liquors,  294 

Divisive  movements  of  intestine,  124 

Dorsal,  definition  of  term,  9 

Driving  force  for  circulation  through 
capillaries,  144 

Drug  habit,  287 

Drugs,  286  ff . 

Ducts  of  glands,  21,  28,  31  (fig.), 
32  (fig.) 

Dyspepsia,  114 

Ear,  structure  of,  255 

Efferent  nerve  fibers  and  impulses, 
75,  78  (fig.) 

Elasticity  of  arteries,  144 

Elimination  of  intestinal  waste,  132 

Emmetropic  eye,  246 

End  organs  of  nerves,  27,  74-82, 
240 

Enjoyment  of  food,  hygienic  value 
of,  115 

Enzyme,  44,  47  ;  action  of,  101  ff.  ; 
of  saliva,  104 ;  of  gastric  juice,  109 ; 
of  small  intestine,  119-123 

Epidermis,  185 

Epiglottis,  20  (fig.) 

Equilibrium,  nervous  factors  in,  272 

Esophagus.   See  (Esophagus 

Ether,  280 

Eustachian  tube,  256 

Evaporation  a  cooling  process,  195 

Excitation,  282,  296 

Excretion,  180 ;  in  relation  to  feed- 
ing, 228 

Extractive,  138,  232 

Eye,  structure  of,  242 

Face,  bones  of,  17 
Farsightedness,  248 
Fasciculus  of  muscle,  33,  34 
Fatigue,  55  ff. ;  hygienic  value  of, 

63 
Fatigue  level,  58,  61 


Fats,  95,  222;  of  meat,  110;  diges- 
tion of,  120-123;  fuel  value  of, 
218  ;  storage  of,  222,  224 

Fatty  acids,  96,  120 

Feces,  130,  132 

Feeding,  muscular  activity  after, 
158 

Femur,  19 

Fermentation,  292 

Fever,  211 

Fibula,  19 

Fires,  open,  213 

Food  accessories,  286  ff. 

Foods,  as  source  of  energy  or  power, 
91,  215;  as  material  for  growth 
and  repair,  91,  229  ;  chemical  com- 
position of,  92,  99,  238  ;  animal  and 
vegetable,  97;  fuel  value  of,  215; 
heating,  220 

Foodstuffs.    See  Nutrients 

Force-pump  action  of  heart,  140, 
142  (fig.) 

Forebrain,  264  (fig.),  279 

Fuel  requirements  of  the  body,  220 

Fuel  substances,  storage  of,  in  mus- 
cle, 48 

Fuel  value  of  food,  215 

Gall  bladder,  108  (fig.) 

Ganglion,    72 ;    of  the   dorsal  root, 

76 

Gas  as  a  conductor  of  heat,  197 
Gaseous  exchange  in  capillaries,  167  ; 

during  muscular  activity,  178 
Gastric    juice,    110;     secretion    of, 

114 

Gelatin,  94 
Glands,  21,  28,  30,  31  (fig.),  32  (fig.)  ; 

working  and  resting,  44,  46  (fig.) ; 

blood  supply  during  activity,  44 ; 

ductless,  28,  64  ff . 
Glottis,  20  (fig.) 
Gluten,  93,  110 
Glycerin,  96 
Glycogen,  225 
Granules,  storage  of,  in  gland  cells, 

46 
Gray  matter  of  spinal  cord  and  brain, 

74,  264  ff . 

Habits,  physical  basis  of,  284 

Hair    and    hair    follicle,    185,    186 

(fig-) 

Harvey,  William,  139 
Headaches,  134 


312 


ELEMENTS  OF  PHYSIOLOGY 


Hearing,  255 

Heart,  10, 12,  21 ;  force-pump  action 
of,  140  ff . ;  valves  of,  141 ;  regula- 
tion of,  159  ;  nerves  of,  160 

Heart  beat,  139 

Heat,  produced  in  working  muscles, 
52  ;  effect  of,  on  the  circulation  of 
the  blood,  152 ;  production  and 
transfer  of,  194,  204  ;  transfer  of, 
from  internal  organs  to  the  skin, 
212;  unit  of,  216;  supply  of  en- 
ergy for  production  of,  220 

Heat  balance,  199  ff . 

Heating  foods,  220 

Hemoglobin,  137,  138,  167 

Hepatic  artery,  24  (fig.) 

Hepatic  vein,  24  (fig.),  26 

Hindbrain,  264 

Hip  bones,  19 

Hoarseness,  21 

Hormones,  89 

Horny  layer  of  skin,  185 

Humerus,  19 

Humidity,  influence  on  temperature 
regulation,  200  ;  influence  on  men- 
tal work,  207 

Hunger,  261 

Hypermetropia,  248,  250 

Ileocolic  sphincter,  128 
Illumination  for  near  work,  251 
Illusions,  optical,  254 
Indigestible  material  in  food,  97 
Indigestion,  114,  132  ff. 
Inhibition,  160,  296  ;  in  the  nervous 

system,  281 

Inhibitory  nerves  of  heart,  160 
Inorganic  salts.    See  Salts 
Instep,  bones  of,  19 
Interdependence  of  organs,  63 
Internal  secretion,  64 
Intestinal  juice,  118-121 
Intestinal    waste,    elimination    of, 

132 
Intestine,  small,  10,  20,  23  (fig.),  25, 

117  ;  large,  10,  20,  127  ;  action  of 

muscular  coat  of,  122,  128 
Iris,  243,  244  (fig.),  247  (fig.) 
Irritability,  45 

Jugular  vein,  23  (fig.) 

Kidneys,   10,   13,   14  (fig.),  23,   25; 

structure  of,  181  ff. 
Kilogrammeter,  216 


Labyrinth  of  ear,  256-258 

Lactic  acid,  48,  59 

Large  intestine.    See  Intestine 

Larynx,  20 

Lateral  costal  breathing,  173 

Lens  of  eye,  243,  247  (fig.)  ;  forma- 
tion of  image  by,  244 

Ligaments,  8,  14,  16,  18 

Lipase,  120 

Lipoids,  or  lipins,  229 

Liver,  10,  21,  23  (fig.),  25,  26,  108 
(fig.),  118  ff. 

Lobes  and  lobules  of  the  lung,  13, 
174;  of  glands,  29,  30  (fig.) 

Locomotion,  nervous  factors  in,  276 

Lumbar  vertebrae,  14 

Lungs,  10,  12,  21,  23  (fig.)  ;  structure 
of,  169  ;  apical  lobes  of.  174 

Lymph,  38  ;  origin  of,  38  ;  environ- 
ment of  cells,  40,  138  ;  gases  of, 
167 

Lymph  flow,  function  of,  40  ;  cause 
of,  150 ;  influenced  by  respira- 
tory movements,  175 

Lymph  spaces,  38 

Lymphatics,  39,  150 

Malt  liquors,  293 

Massage,  150 

"Master  neurones,"  82 

Mastication,  104 

Meat  a  protein  food,  93,  97 

Mediastinum,  11,  20 

Medulla  oblongata.   See  Bulb 

Mental  work,  and  the  circulation, 
155 ;  after  meals,  158  ;  as  influ- 
enced by  climatic  conditions,  207 

Mesentery,  12 

Microbic  life  in  the  intestine,  130, 
132 

Micromillimeter,  or  micron,  136  (fig.) 

Midbrain,  264  (fig.) 

Moisture,  influence  on  temperature 
regulation,  197 

Morphine,  304 

Motor  nerves,  74 

Movements,  active  or  passive,  effect 
on  circulation,  149 ;  respiratory. 
See  also  Breathing  movements 

Mucin,  44,  127 

Mucous  coat,  of  stomach,  108 ;  of 
intestine,  117,  128  (fig.) 

Muscle  fibers,  34,  35 ;  of  stomach 
and  intestine,  108,  117,  128;  of 
arteries  and  veins,  148 


INDEX 


313 


Muscles,  8, 16;  antagonistic  action  of, 
16-18, 209 ;  structure  of,  32 ;  physi- 
ology of,  47,  55 ;  isolated,  49,  56 ; 
and  temperature  regulation,  209 

Muscular  activity,  effect  on  circula- 
tion, 149,  154,  158;  after  meals, 
158,  348 ;  effect  on  respiration, 
178;  and  the  regulation  of  the 
temperature  of  the  body,  206 

Muscular  sense,  259 ;  relations  of, 
to  locomotion  and  maintenance  of 
equilibrium,  278-279 

Muscular  work,  power  for,  217 ; 
feeding  for,  220 

Myofibrils,  35,  52 

Myopia,  248 

Nasal  cavity,  20 

Near  vision,  245,  250 

Nearsightedness,  248 

Nerve  cells,  72  ff.,  268  ff. 

Nerve  fibers,  structure  of,  71  ;  affer- 
ent and  efferent,  75,  76 ;  of  spinal 
cord  and  brain,  74,  268 

Nerve  roots,  74 

Nerves,  27 ;  distribution  to  organs, 
40 ;  structure  of,  71  ;  cranial,  268 

Nervous  system,  general  anatomy  of, 
27,  71,  263  ;  training  by  practice, 
88,  283 ;  physiology  of,  69,  269  ; 
and  the  circulation,  159-162  ;  and 
respiration,  175  ;  and  secretion  of 
•  perspiration,  187  ;  and  regulation 
of  body  temperature,  208 

Neurones,  76;  "master,"  82;  of 
brain,  268 

Nitrogenous  equilibrium,  235 

Nucleus,  31,  35 

Nutrients,  92  ;  classification  of,  93 

Nutrition,  215  ff. 

CEsophagus,  10,  12,  20  (fig.) 

Opium,  304 

Optic  lobes,  264 

Optic  nerve,  28,  243,  247  (fig.) 

Organs,  typical  structure  of,  40  (fig.) 

Oxidation,  50  ff.,  91,  165 

Oxygen,  r61e  of,  in  cell  life,  50  ff., 
165 ;  in  lymph,  165 ;  in  blood 
plasma,  165  ff.  ;  absorbed  during 
muscular  activity,  178 

Pain,  sensations   of,  260  ff.  ;  signs 

of,  281 
Palate,  20 
p 


Pancreas,  anatomical  relations,  10, 
20,  25,  29,  108  (fig.),  118  ;  the 
source  of  an  internal  secretion,  67 

Pancreatic  duct,  108 ;  stimulus  to 
secretion  of,  89 

Pancreatic  juice,  111,  119 

Papillae  of  skin,  185 

Parotid  gland,  29 

Pelvis,  of  skeleton,  19  (fig.)  ;  of 
ureter,  182,  183  (fig.) 

Pepsin,  109 

Peptids,  102 

Peptones,  109 

Peristalsis,  125 

Peritoneal  cavity,  10,  11,  12 

Peritoneum,  10,  11,_13 

Peritonitis,  11 

Perspective,  idea  of,  253 

Perspiration,  composition  of,  181, 
187 ;  secretion  of,  187 ;  sensible 
and  insensible,  187  ;  and  the  out- 
put of  heat,  194-198 

Pharynx,  20,  185,  256 

Pituitary  body,  67 

Plasma.    See  Blood  plasma 

Play,  284 

Pleura,  10,  11 

Pleural  cavity,  10,  11  ;  pressure 
in,  171 

Pleurisy,  11 

Pons  Varolii,  266  (fig.) 

Portal  vein,  23  (fig.),  26,  127  (fig.) 

Position,  sense  of,  259,  276  ff. 

Posterior,  definition  of  term,  9 

Posture,  nervous  factors  in,  279 

Presbyopia,  249 

Pressure  in  arteries  and  veins,  144  ff., 
155  ff.  ;  in  pleural  space,  171 

Processes  of  nerve  cells,  72 

Proteins,  nature  of,  93  ;  chemical 
structure  of,  101  ;  digestion  of,  in 
stomach  and  intestine,  109,  120 ; 
in  blood  plasma,  138  ;  influence  of, 
on  secretion  of  urine,  183 ;  fuel 
value  of,  217  ;  as  a  source  of  fat, 
224  ;  of  sugar,  226 ;  in  living  cells, 
229 ff.;  daily  requirement  of,  235 

Proteoses,  109 

Pseudopodium,  137 

Psychic  secretion  of  gastric  juice, 
114 

Ptyalin,  105 

Pulmonary  arteries,  22,  24  (fig.), 
25,  167 

Pulmonary  circulation,  23, 140  (fig.) 


314 


ELEMENTS  OF  PHYSIOLOGY 


Pulmonary  veins,  21,  22,  23  (fig.), 

24,  169 

Pupil  of  eye,  243 
Purposeful  character  of  reflex  and 

volitional  actions,  70,  79,  271 
Pylorus,  108  (fig.) 

Radiation  of  heat,  212 

Radius,  19 

Rectum,  123,  185 

Reflexes,  80 ;  conditioned  and  un- 
conditioned, 85 ;  of  locomotion, 
etc.,  276  ;  disappearance  of,  dur- 
ing anesthesia,  280 

Renal  arteries,  25,  182  (fig.) 

Repair  of  cells,  229  ff . 

Reservoirs,  arterial  and  venous, 
142 

Resistance  to  the  flow  of  blood,  144, 
156  (fig.) 

Respiration,  organs  of,  21  ;  of  the 
cells,  165  ;  nervous  factors  in,  175  ; 
and  muscular  activity,  178 

Respiratory  center,  175 

Respiratory  movements.  See  Breath- 
ing movements 

Retina,  243,  247  (fig.),  253 

Ribs,  17  ;  action  in  respiration,  172  ff. 

Rice,  polished,  233 

Sacrum,  14,  19 

Saliva,  chemical  composition  of,  44 ; 
secretion  of,  44-47  ;  action  in  di- 
gestion, 104  ff. 

Salivary  glands,  21,  29,  44 

Salts,  inorganic,  44,  96,  232 

Sarcolemma,  34 

Sarcostyles.   See  Myofibrils 

Scapula,  19 

Sclerotic  coat,  242  ff . 

Scurvy,  234 

Sebaceous  glands,  185  (fig.),  186 

Secondary  aids  to  the  circulation, 
147 

Secretin,  89 

Secretion,  general  physiology  of, 
44-47  ;  internal,  64 ;  of  gastric 
juice,  108 ;  of  pancreatic  juice, 
bile,  and  intestinal  juice,  119-122; 
of  urine,  182  ;  of  perspiration,  187 

Segmenting  movements  of  intestine, 
124 

Semicircular  canals,  256-258,  259 

Sensations,  240  ff .  ;  reference  of,  240 

Sense  organs,  77,  242 


Septum,  of  gland,  30 ;  of  muscle,  33 

Shivering,  210 

Shoulder  blade,  19 

Shoulder  girdle,  19 

Sigmoid  flexure,  130 

Sinuses  of  skull,  infections  of,  257 

Skeleton,  14-19 

Skin,  7  ;  structure  and  functions  of, 
184  ;  care  of,  187  ;  as  an  organ  of 
absorption,  188  ;  regulator  of  the 
output  of  heat,  208 

Skull,  17 

Sleep,  circulation  during,  155 

Smell,  sensations  of,  258 

Soaps,  96,  121 

Soda  water,  291 

Solidity,  ideas  of,  253 

Somnambulism,  274 

Speech,  279 

Spinal  column,  14,  17 

Spinal  cord,  gross  anatomy  of,  18, 
27;  structure  of,  73,  264;  func- 
tions of,  270 

Spleen,  10,  25 

Starch,  94,  97  (fig.) ;  chemical  struc- 
ture of,  101 ;  digestion  of,  104, 
112,  120.  See  also  Carbohydrates 

Steapsin.   See  Lipase 

Sternum.   See  Breastbone 

Stimulants,  296  ff . 

Stimulation,  45,  79 

Stomach,  anatomical  relations  of, 
10,  20,  25  ;  structure  of,  107  ff .  ; 
digestive  work  of,  109-116 

Storage  of  material  in  the  cell,  46, 
48 

Submaxillary  gland,  29,  44 

Submucous  coat  of  intestine,  117 
(fig.),  128  (fig.) 

Suction  action  of  breathing  move- 
ments, 148 

Sugars,  95,  105  ff.,  120.  See  also 
Carbohydrates 

Supporting  organs  and  tissues,  36 

Suprarenal  glands.  See  Adrenal 
glands 

Suspensory  ligament  of  eye,  244 

Sweat  glands,  structure  of,  186 

Synapse,  78,  270  (fig.) 

Systemic  circulation,  22 

Systole,  140,  142  (fig.) 

Tannic  acid,  290 
Taste  sensations,  258 
Tea,  290 


INDEX 


315 


Temperature,  influence  of  external, 
on  secretion  of  urine,  183 ;  influ- 
ence of,  on  chemical  change  and 
vital  activities,  189,  190;  of  the 
body,  190  ff.  ;  reactions  of  body 
to  changes  of,  201 ;  "danger  zone  " 
of,  202  (fig.),  205,  205  (fig.) 

Temperature  sensations,  193,  258 

Tendon,  8,  33,  34 

Theine,  290 

Thermal  phenomena  of  the  body, 
189  ff. 

Thirst,  261 

Thoracic  cavity.   See  Pleural  cavity 

Thyroid  gland,  64 

Tibia,  19 

Tobacco,  305 

Toes,  bones  of,  19 

Tone,  arterial,  161 ;  of  skeletal 
muscle,  209  ff . 

Touch,  258 

Trachea,  10,  11,  21 

Trypsin,  120 

'Tweenbrain,  264  (fig.) 

Tympanic  membrane,  255,  258  (fig.) 

Tympanum,  256 

Ulna,  19 
Urea,  181 

Ureter,  181,  182  (fig.) 
Uric  acid,  181 
Urine,  secretion  of,  182 
Use  and  disuse  in  the  training  of 
the  nervous  system,  284 

Valves,  of  heart,  21,  141,  144 ;  in 
veins,  149 

Vasoconstrictor  nerves,  162 

Vasodilator  nerves,  162 

Vasomotor  nerves.  See  Vasoconstric- 
tor nerves  and  Vasodilator  nerves 

Vegetable  foods,  97,  113 

Veins,  7,  23,  26,  38,  39,  144  (fig.)  ; 


intermittent    compression    of,  in 

muscular  activity,  146 
Venae  cavse,  13,  22,  23  (fig.),  24  (fig.), 

26 

Venous  reservoir,  142 
Ventral,  definition  of  term,  9 
Ventricles,  of  heart,  21,  23,  140 ;  of 

brain,  264,  265 
Vertebra,  14,  16 

Vertebral  column.  See  Spinal  column 
Villus  of  intestine,   117  (fig.),  118, 

120  (fig.),  126  (fig.) 
Visual  judgments,  253 
Visual  sensations,  252 
Vitamines,  232 

Vitreous  humor,  243,  247  (fig.) 
Volitional    actions,   or    movements, 

81,  275,  279 

Wandering  cells,  137 

Warm  weather,  circulation  in,  152  ; 
and  feeding,  158 

Warm-blooded  animals,  191 

Warmth,  sensations  of,  258 

Waste  products,  48,  58,  63 ;  excre- 
tion of,  180 

Water,  as  a  food,  96 ;  and  the  secre- 
tion of  urine,  183 

White  matter  of  spinal  cord  and 
brain,  74 

Will,  81,  279 

Wind  and  temperature  regulation, 
200 

Wines,  294 

Winking,  muscular  and  nervous 
mechanism  of,  69 

Work,  unit  of,  216 

Wrist,  bones  of,  19 

Writing,  279 

Yeast,  292 
Zymogen,  46 


THIS  BOOK  IS  DUE  ON  THE  LAST  BATE 
STAMPED  BELOW 


AN     INITIAL    FINE     OF     25     CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  THE  FOURTH 
DAY  AND  TO  «I.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


JAN  13  1933 
APR  13  1933 


27  1933 


PO1936 

OCT231936 
NOV  18  1939 


LD  21-50?n-8,'32 


YB  795!  7 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


